Here's the issue I have with saying that Imposter Chansey is broken. Yes, it can take any member of your team and make it crazily strong bulk-wise. However, it needs to be understood that there are a bunch of viable ways to 1-on-1 Imposter-proof your Pokemon. Some examples include, for those of us who don't already know:
- Judgment shenanigans
- PH alongside moves that you can tank and heal off (but with which you can wear down their non-healing Imposter)
- Safety Goggles+Spore
- Leech Seed w/o other recovery
Now, of course, not every set can viably incorporate one of these techniques (-ates come to mind). However, what must be recalled is that Chansey is not always going to be in 1-on-1 against you. Pokemon is a team game, and as such you have team support. In fact, dealing with the threat of an Imposter Chansey in general is pretty easy when teambuilding because it has 0% unpredictability in its moveset. You literally get to prescribe the Chansey's moveset for it.
Take, for instance, a hypothetical situation where I have an Aerilate Rayquaza-Mega with Fake Out/Extremespeed/Boomburst/Magma Storm. If I run a Soundproof Slowbro-Mega or Flash Fire Registeel (or anything else that walls that set) alongside it, I can fearlessly bring out my wall against the Imposter. Walling opponents in BH is always tough because there is pretty much always a move that fucks over your wall (think Techno Blast for Slowbro-Mega or TA for Registeel). However, when taking on an Imposter you know
exactly what their capabilities are. Of course, they now know what yours are in return, but that's beside the point of whether the Imposter itself is necessarily broken.
It's also worth noting that, assuming you have a team with a semi-decent defensive framework, you should definitely have defensive checks to your own offensive Pokemon because
goddamnit, your opponent could be running one too. Running an Aerilate Crayquaza and complaining about getting fucked over by Imposter Espeeds is stupid, because Imposter Espeeds are almost 100% certain to be
weaker than those fired off by the original Ray (most Rays run a boosting item, whereas basically all Chanseys don't). You are playing BH, you need an -ate check. Deal with it. Yes, it is extremely difficult to break an Imposter Chansey with Eviolite intact via sheer force, but get creative. If you can wear the Chansey down with Status, Rocks, or the paltry damage that comes from attacking it, it'll die soon enough.
(Note, this whole spiel was written with the idea that Chansey would be Impostering an offensive threat; I haven't really looked at copying support/defensive threats yet, but it seems to be a relatively weak reason for banning it. Feel free to prove me wrong, though)