They are all QB's that, despite being pretty high-level recruits, did not play at a satisfactory level at Notre Dame. The proof is in the pudding. Look at their level of play at the universities that they transferred to. Not one was successful. Buchner has not been successful either and from watching his play this year and last year, won't be successful in the future. It's not BK's fault, nor anyone else's. They just did not pan out at the college level.
This happens everywhere (DJU at Clemson for example) and at every position. Unfortunately, ND has had more than its share of QB busts. As per it being BK's fault, you might want to ask Jayden Daniels how bad he is with QB's. Daniels has improved immensely from last year. That isn't to say that Kelly is a QB guru because he isn't. But the busts at QB at ND were not his fault, nor TR's or anyone else's. It just happens.