Don't debate semantics, broken is just a descriptive term. Walls are considered broken in smash because every stage that allows infinite combos (sans Stadium, which aren't infinite due to the walls only being up for 30 seconds) has been banned in this game and people STILL can't agree on Wobbling.
I've been trying hard to be civil while you act intentionally ignorant, and ignore entire swaths of posts just because you have no argument to refute them (see: play stops for 30 seconds at a time because of transformations, the entire left side of the stage argument).
You quoted "Whoever starts on the right side of the stage has an advantage because all they need to do is defend their area. They don't need to approach because their opponent will eventually be forced to attack them or move to the extreme edge of the stage once the mountain shows up." and post
Why is the left forced to attack the right? You are giving no cause.
immediately after it like you don't understand that being on the inside or on top of the mountain is a weak position in the vast majority of matchups, leaving the left side with the option of "approach the right" or "fall back to the other side of the mountain". You either believe the inner part of the left stage is not a bad defensive position on the fire or mountain transformations or are being intentionally obtuse.
Let's say it's Falcon on the left and Marth on the right, Falcon has to approach Marth or be forced to the side. Same with pretty much anyone vs Marth, Marth gets an advantage because he started on the right side of the stage and knows his opponent has to approach him or eventually move to the edge (or into a position he can easily hit them in). Spacies also have a similar scenario because almost nobody wants to go into the center of the mountain where they can get infinite comboed into a wall. Is it a minor advantage, and one that not many people even recognize? Yes it is. Does it exist? Yes.
I want to point out some things you've said.
First quote's context is you talking about why the left side isn't at a disadvantage:
Why is the left forced to attack the right? You are giving no cause. The stage change itself doesn't make one move forward the same way brinstar or RC force you to jump. The player on the left side would only have to move right if he chose to.
Second quote: Sveet, on why the left side of the stage is weak in the mountain transformation (former edgeguardee is the person who is on the extreme left of the stage, edgeguarder being the one further to the right)
I tend to just trap them to the inside of the mountain and defend and look for openings as they come. If both players stand off the whole time, the only advantage gained by the former edgeguardee is feet on the ground, back to the edge. That is no different than if they used randall or edge canceled or shortened an illusion or you messed up or they guessed right and got through your trap. This is a very minor positional gain with primary advantage still with the former edgeguarder.
Bolding is mine. Also, it's different from Randall or a shortened illusion or anything like that because those things aren't the stage. Having feet on the stage is a big deal, it means you have all your jumps, the ability to shield and all that it entails, along with the ability to CC, dash, whatever else you can do while standing.
If neither player attacks for the first 90 seconds and the first transformation is the mountain or fire stage, the player on the left is forced to attack or move closer to the left edge of the stage due to no fault of their own because of how weak the inner parts of the left stage are during these transformations. The right side is in a superior position because the stage cannot randomly screw with them, all they need to do is defend their position until a transformation occurs. That transformation then has a 50% chance of giving the left side a disadvantage.
Unless the platforms, top, or inside of the mountain/fire tree aren't weak defensive positions you have to concede that the left side has to make an aggressive move or end up being forced into a weak position for reasons that are entirely unrelated to what their opponent is doing.
You can say the problem isn't a big one and that Stadium is worth it, but you can't ignore that the stage has problems.
Here's the list of known problems with Stadium:
1. The mountain coming up during an edge guard is a thing that randomly happens and favors a certain player. The grass stage is similar, but some characters do not benefit from a sloped edge.
2. There is a glitch on the stage that, though rare, can make players fall through the floor and lose a stock.
Debatable problems:
1. The flow of the game is broken on certain transformations because of the strong defensive positions that are created.
2. The left side is at an inherent disadvantage because they have to be the aggressor or eventually be forced into a worse position unless they have a means of forcing the opponent to engage (projectiles). One player starts on the left side, thus that player is at a disadvantage from the start of the game depending on the matchup.
3. Some transformations have walls, and stages with walls have all been banned with the walls being part of the reasoning behind the ban.
Argue against the ones that are up for debate, and argue why the ones that are known are not a big deal. Anecdotes are not arguments.