I think questionable is the fairest option. He's shown a good chin on other occasions. However not getting countered is part of having a good chin and he seriously ate that shot like it was the last mince pie at christmas dinner.
You need more than a good chin in boxing you need some good defense to go with it and without that that good chin gets chipped away little by little till it’s glass. Frank the animal Fletcher is a good example.
Good chin, horrendous defense. Even if a guy has a low ko%, a shot with perfect technique and timing can still ko almost anyone.
If a bald man dons a toupee to cover up his baldness and then someone gives him a big slap across the chops and said toupee goes flying off would anyone question if he was bald or not?
He's got a questionable chin now. I dont think that punch would have put Canelo down TBH. But all credit to Burace that was a perfect right hand, there was no luck about it.