Yeah, to be honest I'm of the opinion that all the managers (barring Moyes) did alright, for a time, relative to their own individual coaching ability.
- Moyes was shit, though firmly on him IMO. Not a top tier manager, so why expect him to succeed at a top club?
- Van Gaal finished 4th, implemented his system (just was boring as feck), and then won the FA Cup with a very young squad and a group of very meh players as he was doing a huge overhaul. He probably wasn't capable of being a title winning manager in current day, because hes too stubborn to adapt to other players so he wouldn't be able to work with good enough players on a daily basis
- Mourinho isn't a top manager and hasn't been since his Madrid team. Had 1 year with Chelsea after. At United, he still had an encouraging 1st year and then 2nd year finished 2nd to Peps 100 point city and reached a cup final, hardly an embarrassment, but the 3rd year also was a standard Mourinho 3rd year collapse (and after his time here has shown he's not a top manager anymore).
- Ole finished 3rd and 2nd and had multiple good cup runs. A poisoned chalice is not a job where essentially nobody managers can come in and do that. He was never at the level to compete with guys like Pep/Klopp/Tuchel etc, but he did pretty decently until it was time for a step up and he couldn't do it.
From a football perspective, I say it a lot, but our biggest issue has been just poor manager selections, and expecting average managers who don't implement specific systems to compete with great managers and great coaches.
England as well. They need a coach. Southgate doesn't have a hope in hell coming up against good groups of players who also have good, progressive coaches (Netherlands, Germany, Spain).