The self-inflicted wounds in the late 80's probably take the cake for the bad thing. But I was on the younger side back then, so I'll stick to more recent events:
Good:
1) Walt Freaking Harris. He took a team that had been winning 3.43 games per year in the 90's and carried us to respectability. In his last five seasons, we averaged 7.8 wins per and had a winning record in each. He created the floor that we have even today. Pitt has never won less than five games in a season since he left. He was completely underappreciated here. Perhaps a victim of his own success.
2) The extra Covid year. Funny how thin the line can be. Narduzzi is the king of the castle right now and probably a lifer here. Take away Kenny Pickett's historic season last year, and there's a very real chance he gets fired after the season. We were 6-5 in 2020, and we simply were not getting over the hump. I'm sure plenty of people will deny it, but the sentiment at that time was not good. Even after the Western Michigan game people were turning/tightening the screws on him. But we had a huge advantage last season in the form of experience.
And then a runner-up here might be the losses to BC and NC State in 2020, because there's a chance Pickett walks away feeling satisfied if we got 8-3 (8-1 with him as a starter) that year. I know he mostly came back to bolster his draft positioning, but you just never know. In that case (8-3), I don't think Narduzzi would have been on the hot seat, but I also don't think we go 11-3 and win the ACC last year.
Bad:
I mean, it has to be firing Wannstedt. Honestly, I don't even think he was that great of a coach. But we needed to look at the big picture: He won 27 games in his final three seasons and was recruiting at a level that was going to keep having a breakout year in the realm of possibilities. Losing to Cincy and Uconn in consecutive seasons when both would have given us the conference sucked. But, like I said, it's all about the bigger picture. And if that SI article had any influence on them, well then shame on the board. I won't pretend to guess how much or how long that set us back. But we averaged 6.7 wins/year in the ten years after Wannstedt was here. I'm willing to be that would be closer to 8.7/year had we kept him.