If you don't mind, I'm going to use your question as a jumping off point for a much broader point I want to make about this whole discussion, and why this underlying sentiment of
Frihetsanka
vs. Smashboards seems to be so prevalent. I'll first start by addressing the actual question, as to why Umbra Clock Tower was banned.
Consider the then legal stages of Smash 4: Battlefield, Final Destination, Smashville, Town & City, Lylat Cruise, Dreamland 64, and Duck Hunt (which was banned briefly after Bayonetta rose to prominence). Of all these stages, the only ones that have anything resembling dynamic platforms are Smashville, T&C, and Lylat (unless you count the rising dog on DH as a dynamic platform.) Smashville has a single moving platform that has a consistent, predictable pattern of movement. The time it spends offstage is 3-4 seconds before returning to hovering over the main stage. This is usually seen as just enough time to set up a creative kill or method to camp without being considered broken, because it last no more than four seconds. Town & City has its two transformations cycle every 30 seconds, and only the second has platforms that ever briefly go offstage. Otherwise, the movement of the platforms is both over the main stage and within a low enough jumping distance that heavies and Little Mac can reach them without expending their third jump. Also consider the fact that despite all this, people legitimately began considering banning the stage due to the stretcher platforms being able to kill mid-transformation, and this sentiment has only increased since Ultimate's hazards off version has those second transformation platforms permanently offstage. Lylat has the entire stage tilt, which is its own kind of movement and was already seen as unfavorable by much of the competitive Smash community, only being kept around because they NEEDED five legal stages.
Now, look at Umbra Clock Tower. Aside from the base form which is similar to Final Destination, the stage includes seven different randomly transforming platform layouts, of which three include permanently offstage platforms, one includes a moving cave of life, and two include asymmetrical layouts. Though none of the transformations (aside from Platform Layout 6 imo) can be viewed as game-halting, and is tame even compared to Pokemon Stadium, which is still legal in Melee, the fact that none of the other stages include elements that allow these strats to be used at all, even temporarily, causes the stage to be viewed unfavorably. Also consider the fact that these transformations do not follow a set pattern, unlike Town & City. This becomes an annoyance to players as planning ahead to utilize a specific transformation at a specific point in the match is no longer a viable option, and the players must instead adapt to the stage around them. All of these things combine together to create a stage that simply does not fit what is usually viewed as the competitive meta of Smash 4.
Now, you may be asking yourself, aren't these transformations temporary anyway? If a player decided to camp on one of the more dangerous transformations, isn't it alright since the transformation is on a time limit and the player will eventually be forced to return to the main stage? The answer to that, theoretically, is yes. But see, the more involved, dynamic nature of this stage immediately sets it apart from Smash 4's five/six legal stages, and these elements contribute to the stage being viewed as inconsistent, or "janky." The Smash Bros. meta, ever since the tail-end of Brawl's lifespan, has been pursuing a meta that is safer and more consistent than competitive Smash had been in the past.
A lot of debating in this thread has centered around how the starter/counterpick distinction is a flawed concept that only serves to create stages that don't get practiced on and eventually get banned anyway. This has historical precedent in its reasoning. Early Melee stagelists included Brinstar, Corneria, Green Greens, Jungle Japes, Kongo Jungle 64, Mute City, Poke Floats, Princess Peach's Castle, and Rainbow Cruise all as legal counterpicks, until issues reared their head which threatened tournaments' competitive integrity. Since then, Pokemon Stadium got moved from a starter to a counterpick, and has mainly been kept around exclusively because of its historical longevity as a stage in competitive play and players' familiarity, though now is beginning to see a push to ban as well. Brawl, by contrast, had a much less agreed upon set of stages for competitive play, and saw stages like Castle Siege, Delfino Plaza, Frigate Orpheon, Halberd, PS2, Brinstar and Rainbow Cruise as relatively common stage picks, and even then, some regions were even more liberal with their allowances, including stages like Bridge of Eldin and Pirate Ship as legal stages.
Many of these stages stayed for decent periods of time into their respective games' competitive lifespans, with many of Melee's being utilized by as late as 2006, five years into the game's life. As more stages became banned however, an attitude shift in the community sought to "normalize" stagelists by only including the safer, more static stage layouts as legal. This became very apparent at the beginning of Smash 4's life, which decided to cut many "fringe" stages like Skyloft and Wuhu Island off the bat and only play on the safer choices. This change in the competitive mindset has changed the way in which stages, and to a certain extent, Smash Bros. in general is viewed. Plugging in stages like Skyloft, Wuhu, and Umbra into the current stagelist for Smash 4 creates a shakeup in the meta that de-emphasizes consistency and rewards environmental adaptation, or stage knowledge, if you will.
This is why I find a lot of the "testing" that many TOs and top players claim they will do for stages in Ultimate to be somewhat disingenuous. Honestly, the truth of the matter as to why we just can't seem to see eye-to-eye on this subject has a lot to do with our competitive upbringing. To address
Frihetsanka
specifically, notice that he considers 12-16 stages as a liberal amount of stages, whereas many people in this thread are arguing for 30, or even 40. I want to try to understand his perspective and allow it to inform the rest of you, so let's try a little exercise.
Let's work under the presumption that 12-16 stages is indeed a liberal amount, and a good number of those stages are bad anyway and will quickly become banned. What then would be a more normal stage number? Let's work with 7. So five starters and two counterpicks. Sounds reasonable from this perspective. The five starters are nearly guaranteed to be Battlefield, Final Destination, Smashville, Pokemon Stadium 2, and Lylat Cruise. These stages all have relatively similar blastzones, small-medium stage sizes, whole shebang. Now what qualifies as a counterpick? Under the commonly understood definition, these would be stages that provide a slight edge to a more specific playstyle. So let's go with some safe options, WarioWare Inc. and Kalos Pokemon League. Not bad, right? Well, except for the fact that WarioWare has much smaller blastzones than the five starters, AND has a walled underside allowing for walljumps, which none of the other five stages allow. Suddenly it looks a lot less consistent. And Kalos, despite technically being a unique stage layout, has two platforms a similar distance away, like PS2, and also has a walled underside. This could, by some definition, be seen as a PS2 echo, which overcentralizes a meta with only seven stages. This would likely be precedent for these two stages to get banned in the end. Other stages have even more deviations from the five starters. T&C has an entire 30-second transformation (of two!) which has offstage platforms, leading to better camping ability than the five starters. Yoshi's Island is just a larger hazardless Smashville with a walled underside. Too similar. Skyloft has a sharkable main platform, which you can't do anywhere else. Frigate and RC both have a wall. Arena Ferox, Mushroom Kingdom U, and Wuhu Island are much bigger than any other stage. And you can forget about any transforming stage having a shot at legality. They have walkoffs, and that is never okay, even temporarily!
When you start to see stages under this light, suddenly a lot of stages seem to look inconsistent, and the counterpick category begins to look shoddy in general. Why did we ever even allow counterpicks in the first place? No, we as a community have had our standards grow since our silly bouts back in the mid-aughts, and now we can REALLY see what constitutes a proper legal stage in competitive Smash Bros. Now, let me be clear, I do agree with a lot of the stage bans that eventually happened in Melee, as they really did cause a lot of matchups to become overcentralized. However, I feel like the environment that this reasoning created got severely out of hand, and it makes me question the intentions of players who hold these perspectives so strongly. It makes me question why it simply isn't universally agreed to play on one stage exclusively. After all, Smash 64 holds this ideal as their meta, and they get along just fine. This would allow the sometimes lopsided nature of Battlefield, Final Destination, and Lylat Cruise to no longer have a stranglehold on the competitive community, and simply do games on Smashville or hazards off PS2. This would increase consistency and reduce the potential jank that can occur during a match.
What we're looking at here is a perfect example of culture/generational clash, and the reason why this is so pronounced here on Smashboards has a lot to do with the site and its history in the Smash community. Smashboards was the first major discussion platform for competitive Smash Bros., spanning all the way back to the early Melee community for sharing tech, matchup knowledge, and tournament results. Since then, community discussion has shifted to more modern, quick forms of communication like Discord, Twitter, and Facebook. That means that the people who still use Internet forums are people who have been with the competitive community for a long time; ergo, their ideals also hold true to what was considered the standards for those times. You only need to look at a user's join date to quantify that much. The intense desire of the Smashboards (and Reddit, to a lesser extent) community to see a stagelist that reaches 30-40 stages speaks a lot to what we wish we could have had when we played Melee and Brawl competitively, and now see an opportunity to make reality. However, this stands in opposition to the newer community who place predictable and safe stage interaction above all else, and include many of the players who rose to prominence in Smash 4. Of course these newer opinions are going to hold more weight; they are who currently consist of the top of the Smash community.
So after this major wall of text, do I have any conclusions to draw? To be honest, the only thing I can really say is that the competitive Ultimate community doesn't truly exist yet. Like every Smash sequel, it will likely draw much of its competitive ruleset philosophy from the immediately preceding Smash game, and will evolve from there. It is entirely possible that the players we don't even know exist yet will end up preferring one style over the other. Which style that is, I can't say for sure. This is why the beginning of this game is so important to me. If we act rashly and hold fast to the ideals that grew with competitive Smash 4, it will color the entire competitive experience of Ultimate irreparably. I urge all of you to think about what you really want the competitive landscape of this game to look like, and act accordingly. We've only got one shot at this.