I’m going to try and approach this question objectively as possible.
1) He is not a high level coach, which immediately would bring up some red flags. Coaching seems to have become a very big factor in football these days with the rise of sports science. Man-management seems to have taken a backseat to coaching these days. Coaching players to play a certain way and making them disciplined is more important than motivating them and telling them to just do it.
2) He doesn’t seem to have a good network of coaches to tap into, judging by the fact he didn’t change any of the original coaching staff from the Jose Mourinho regime. It was basically ancillary coaches from the Ferguson days like Phelan, Hartis and Dempsey… and while it’s a meme on the site, Ralf in the nicest possible way said that he doesn’t know what Phelan did/does at Utd
3) He doesn’t seem to have a good knowledge of the player market for transfers and it seems very limited. At Utd, he bought the obvious choices (Maguire, AWB, Sancho) and took a punt from his former associates (Giggs recommended James). At Cardiff, he tapped into the Utd academy (his old stomping ground) and bought in a bunch of players from Norway (where he had been previously)… and also some downright weird transfers, like spending £40m on someone who never plays even though he surely okayed the deal (I understand Utd have a more committee mindset to transfers since Jose but surely he has input into transfers since he’ll be the one using them)
4) He cannot implement a clear, competent style of play. He talked the talk but either he isnt good enough at implementing the system he wanted with his coaches or he straight up lied… some of his decisions contradicted themselves. He wanted to play high line but buys the slow Maguire for £80m. He wanted to use full backs as integral to attack but buys AWB who is by no means a great attacker.