Going to also back up Adrian here - Imposter is mandatory to check, be it by using judgment or changing your movesets to accommodate the introduction of one move. One could put forward the argument that if you're weak to that kind of set that your team has an exploitable flaw, but you have to remember that Chansey can almost always stay in as long as it likes with its massive bulk, which is unlike other mons. If by having chansey the opponent can wreck your team, you will usually be left in a bad position.
While defense is still very viable, the metagame is more weather-offence oriented now than it ever has been while stall has fallen out of favor with the introduction of absurdly strong weather abusers.
Finally, while unaware is very common nowadays, set-up sweeping is far from struggling. Anti-unaware moves, such as taunt and gastro acid are very real possibilities on non-contrary sweepers, and contrary sweepers can usually use trick, crippling the unaware user for the rest of the game.
