Tom Donahoe had been a highly appreciated personnel man before joining the Bills. Yes, he didn't have great luck after joining the Bills, but I think John Butler had orchestrated a few bad drafts before his arrival that made Donahoe look worse than he was.
According to many on this board, his primary failing was not finding a NFL starting QB. Not many were coming out of college at that time, as I recall.
I am not here to say that Donahoe was very good, but there usually are extenuating circumstances when a highly regarded person fails in a new situation.