He was handed a large number of very high draft picks, and accomplished almost nothing. He's had Kyrie and other FAs walk for nothing, something Neil has been crucified for. They should have been the best team in the east for years now, and yet they are underachieving.
I like Ainge, he's my generation, and does a good job of representing my state (Oregon) and my generation. I openly root for the guy, and always will. However, he's just not a good GM. He wanted to abuse anyone trading with him by getting the far better end of the deal. It rarely worked out, causing him to "become stagnant," and his franchise to underachieve. I'm very sure that if you were to ask Ainge himself about his GMing, he would say that he'd do things very differently if he had a chance to redo it.