Overall I never considered him a bad coach but when the context relates to Barcelona there is no denying that he failed there.
Koeman is a pretty good coach but not in La Liga and Barcelona because indeed some of his decisions at Barcelona are considered controversial and this makes him even worse in the eyes of the fans. But for achievements outside of clubs other than Barcelona, he is clearly a good coach.
Koeman did alright when he was there, the point is that he is not the coach that would have a good system with Barcelona, that's the problem. I mean think about it this way, if there is a better team than Barcelona and you could manage it better, but you would fail to manage Barcelona, that is not because you are a bad manager, that is because you are not a good fit, that's all.
And I believe that Xavi is a great fit, he is the "son of Barcelona" type of manager and he will do his best to lift that team higher. This doesn't make Koeman bad, it just makes him not fit for Barcelona, he could still manage another team and be great there. For example PSG could get him if they fail to get zidane? That could work.