Well, here's my point of view: it's not normal for a Pokémon to be used by 50% of OU players. And we're talking about Chansey, I mean... Natural Cure? Not a problem. 250 HP/105 special defense base stats? Not a problem either. But the complete combo of switching out and healing its status problem, being almost 'un-OHKO-able' (did I just make up a word?), healing a 50% of its maximum HP, and using just one boring damage dealing attack. What's the point of all of this?
You can't play anymore the way you want to play, you can't use anymore the Pokémon you like to use, because if so, you already know you're probably going to lose against Chansey. You have to create a team composition while thinking 'what should I use to beat Chansey?', and it takes SO away the originality and versality of the game. It's not anymore about using a decent composition, having coverage, trying to fit UU/NU Pokémon in OU tier. It's all about beating Chansey. And that's the worst part: Chansey's users make you to be like them, automated players who don't think at all and use a Pokémon just because it's broken, otherways, you won't won anything. And that's not even 'playing'.