This ability is genuinely uncompetitive in OU. In standard 6v6 Smogon singles the ability to deny meaningful damage progress by simply switching out thus even rewarding potentially bad switches in some way is way too braindead and exploitable due to the nature of switching often being the best defensive answer to something. It's especially bad because so many Regenerator Pokemon form degenerate Regenerator cores with each other.

It should be banned altogether from the tier. No, I don't care that Corsola and Audino suck. A busted ability can't redeem every possible shitmon ever.