My thoughts on Umbra:
On every single transformation, after the platforms disappear, you're left at a disadvantage if you camp on them. You're either a sitting duck, falling in the air (if the platform goes up) or suddenly in a state in which you need to recover off stage (if you're camping the sides).
Thus, camping on this stage isn't an auto-viable strategy. Thought has to be put into it as to whether your opponent can outplay you in your disadvantaged stage or not (thus, retaining the game's concept of winning via skill and not "jank"). The fact that each transformation lasts only 15 seconds, at most, helps a bunch, as they can't wait there forever (thus, putting themselves in a disadvantaged state quickly). A smart player won't challenge the camper, especially if they don't have easy ways to do so, such as a command grab, because they know that as long as they're patient, they're rewarded with stage position and an advantage. With this mindset, the reward will happen every time someone attempts to camp, and because each transformation comes and goes so quickly, camping isn't viable if the opponent is aware of their conditions. Thus, every single time, the more skilled player will win this exchange (recognizing when to camp vs being patient).
I can see this stage being a counterpick for characters who like to have a brief moment to recompose themselves, whether it be a Rosalina wanting to take a couple seconds to get back a Luma or a charging projectile character who wants to grab a quick boost before they get back into the fight. The platform arrangements make it hard to approach anyone on the obscure ones, without say, a command grab, but staying on the platform is too much of a commitment as well, as you become telegraphed (staying on it until it disappears puts you in a position where you're forced to go away from it, because the platform no longer exists, as opposed to jumping off of it in an unpredictable manner when it is still visible on screen, as the opponent is speculating whether you are going to remove yourself from the platform or not). Because of this, it benefits those who need a quick 2 or 3 seconds to get an advantage.
Also, as noted in many discussions before, it also benefits characters with mediocre recoveries.
--
Basically, I believe that this stage should be legal. Camping on this stage have risks to their rewards, much like the touring stages in past rule sets, and as stated by many, after a few games on this stage, the "distracting visuals" become far less distracting.