I don't think there was a single good decision made with the Xin changes.

First: Xin wasn't a bad champ, he wasn't really hurting for a buff.

Second: Xin is a sticky melee auto attack based champ, these kinds of champs seem to be hard to get in a sweet spot to begin with, it's really common for them to be either useless or way to strong any time changes happen, so when a champ like that is balanced, it seems best to give SMALL changes if any at all, not new scalings AND new mechanics at the same time.

Third: self heals are EXTREMELY volatile on melee auto attack champs, a change is heal amounts is pretty much THE MOST LIKELY THING to break one of them. a change in healing is what lets Xin now and the ol 420 weedwick stand in the middle of 3-4 people and just out sustain their damage, forcing you to 100-0 them under CC or just lose. (in Xin's case, literally duel turrets as well).

It feels like at every major junction in the decision making process they made the wrong choice. Working on a champ that didn't really need it, giving big changes to a champ that should only get small ones, and making said changes in the worst place.