Out of the previous suspects, I'd say for me only the last 2 were the ones that weren't clear-cut and required a bit of thinking. In the case of Illusion, there are valid arguments that could be made both for and against banning. I'll go over the most important arguments and offer some counter arguments for each of them and leave the decision to the ones that have made the reqs.
The most valid arguments for Illusion being uncompetitive stem from a. Illusion being something you cannot expect from team preview (except in cases such as Kangaskhan) and b. even if you do expect Illusion there is no way to see it coming if the user plays correctly and c. In the case of Red Orb Groudon (and other Illusion users that are holding Mega Stones), Illusion serves an additional purpose (in the case of Red Orb don, this is the only purpose that Illusion serves since DesoLand and the message saying that the mon is undergoing Primal Reversion makes it quite obvious that Pdon is on the field) that is also pretty strong: completely Improofing the mon without the need of any team support (in this case, damage will not break the Illusion so Imposter can not work on it until the mon switches out), and finally, d. The player using Illusion can play mind games with the opponent by bringing in the Illusion mon disguised as an "opposite" wallbreaker. For example, the opponent could have a Pixilate Mega Diancie and an Illusion Mmx. If they bring in their Mmx disguised as Mega Diancie (or if they just bring in Mega Diancie), the opponent would be forced to go for a coinflip play (given that the opponent has no way of telling the 2 mons apart) which would imply that Illusion is uncompetitive. I will not be bringing up the Illusion-related bugs as valid arguments for banning the ability since the existence of the bug is not really something that would make the ability banworthy, it just means that PS has some bugs that need to be fixed. Now for the counter arguments:
a. This is a pretty good argument, however this argument also implies that Illusion is only broken in tournament settings and not the ladder since you can usually face the same opponent multiple times on ladder (especially in high ladder games where the number of players in that elo range are low) which means that you know what to expect the second time around. So now assuming that Illusion is uncompetitive only in a tournament setting, let's look at why or why not something that is unexpected from team preview could or couldn't be broken. First off, we have to acknowledge that Illusion, while on paper is not something you can "see," is still something you can anticipate/mark/play around. As already mentioned in other posts, if you see the opponent pivot out to bring in a "Giratina" against your Registeel, more often than not it's actually Mmx or a similar mon that can KO Registeel. Additionally, Illusion users are extremely susceptible to getting "marked," either by hazard damage (for example, if the "Giratina" takes 6% from Stealth Rocks), their HP (once an Illusion user gets marked, that same mark can be used to identify them until the end of the match; for example, you will know that the Illusion Mmx is at 94% HP so even if the hazard damage taken by the Illusion user makes sense, their HP might not), or any status inflicted on them (for example if you burn/para the Illusion user you will be able to track it for the rest of the match. Additionally, if the Illusion user comes into the field disguised as a mon you know should be statused, that would be another giveaway). So in conclusion, Illusion is not something that has 0 counterplay or something that any player could use effectively for that matter as there are many things the Illusion user has to take into account and play around in order to make it work. I will however also acknowledge that not every Giratina that comes in on your Registeel is an Illusion Mmx, and that if you make the scout play you will more often than not end up losing momentum. But then again we are discussing tournament play and honestly making safe plays such as this in a tournament setting is completely acceptable and even expected given that the players in these kinds of games are assumed to be competent.
b. This argument brings in an additional element to the table which is player competency: "If played correctly, Illusion would then be a broken ability." What this implies is that if the Illusion user plays in a way that their Illusion mon doesn't get marked or give anything away, they would then be able to claim the key KOs they need in order to win the match. This is valid to an extent, but there still are some weaknesses present in the argument. First off, there are some things that are out of the Illusion user's hands when it comes to the match. For example, the player cannot always play in a way that the last mon in the party is the one they want when they need to bring the Illusion user into the field. To explain this in a more detailed way, let's say the opponent has brought in their Registeel on your pivoting move. Naturally, this means that it's the perfect time to bring in the Illusion Mmx and get a free KO on the unsuspecting Registeel, but at the same time you realize that the last mon in your party is your PH Giratina which has already been revealed during the match. So if you decide to bring in your Mmx at this time disguised as a non-statused Giratina, your opponent will definitely know what is going on. This is obviously just an example as there could be many other scenarios that would result in the same problem in situations such as this.
c. This is a good point, however imo I feel like this aspect of Illusion, if looked at in a vacuum, is not broken at all. If all Illusion did was give the user a way to be completely improof until they switch out by forgoing an item slot (Mega Stone) and potentially the ideal ability (Mega forms do not necessarily have the optimal ability for that mon), I would say that it's definitely a fair trade. Often times we improof our own mons by doing similar things (forgoing item slots for plates, using less than optimal abilities such as Unburden, and the list goes on). So in conclusion, using the improof aspect of Illusion to justify the banworthiness of the unexpected aspect of the ability does not seem like a sound argument.
d. This is the only argument out of the 4 that successfully justifies Illusion being uncompetitive. However, I will mention as a side note that in order for the scenario mentioned in this argument to happen, the user still needs to overcome multiple obstacles: they need to keep the Illusion mon undetectable (by any method available to the opponent), make sure that the non-Illusion breaker is the last mon in the party when they want to bring in the Illusion user, and make sure they can repeat that process multiple times to increase their chances of the opponent finally making the incorrect coinflip play so that they could get their KO. In other words, while this argument is completely sound, it still isn't a risk-free thing. The player using Illusion still needs to play in a way to make sure that the opponent eventually slips up and guesses incorrectly. If at any point during the match the Illusion mon gets marked by any of the methods already mentioned, the Illusion user has pretty much lost their wincon (given that the opponent has an answer to both of the wallbreakers).
After going over the arguments, my personal conclusion is that if the ability even has a chance of being uncompetitive (case d), then it should be banned regardless of its success rate. Even if case d happens 1% of the time, regardless of whether the Illusion user won the match or not, Illusion was still an uncompetitive ability. For that reason I think that the ability should be banned.