there are ways to builds that can break these, primariy physical fire/guts can break it. Secret sword is also quite effective, not many users for that though
A burned guts conkeldur can 2HKO easily with drain punch, infernape can kill it with close combat as well(or contary close combat if that's your thing). It is far from invincible but giving everything ideal abilities tend to boost defense more than offense, do to the existence of prankster with sableye's move pool, on specialy deffensive things. Gut's is a really good ability to beat Chansey,magic bounce stops it cold and it's not like these things have bad move pool's either,
you have absol with sucker punch,boosting, and a ton of coverage(including fighting),
Sabeley has more variety in boosting, two types of priority one of which is sucker punch ridiculous coverage options, and pain split, recover, and status
Even xatu has sucker punch, along side drill peck for those who want to give it too a bird, and u-turn to punish switches. it has wish for support as well or roost for more instant recovery
funny how most magic bouncers have sucker punch?
these arn't eve niche options either gut's is basicly a band in return for passive damage, magic bounce is amazing, and physical fire types aren't exactly unheard of either(combine that with contrary super power's running around and their are plenty of things that can break it.
why would you ever use chansey(for transform) when Blissey is an option for transform?