The problem here is that it's all just too slow, De Boer was the same at Palace, he was the same at Inter, LVG had Utd playing at a walking pace.
My thoughts are that the Eredivisie is quite a slow league, which is partly why a lot of players have come from there and failed in England. Technically it's a different style, but in the PL to be successful you really have to be faster technically and physically as a team.
Pep's game while based on those ideals is much faster and more fluid. The ball moves quicker, he uses this quicker ball movement to drag teams to one side to create overloads on the other. Being able to change the speed of play is crucial, his teams will go from walking pace to break neck speed in one or two movements. The quality of passing is also huge factor, the speed, the weight of pass, always to the players right side etc. All that stuff makes a huge difference when done at pace, especially with players not having to break stride or check movements.
ETH has come directly from Eredivisie, a slower paced league, with him on the ball Utd are too slow, ponderous and too sloppy. Players don't contribute to creating good angles and off the ball movement seems like an afterthought. Rather than creating space by movement, players often look like they are waiting for someone else to do it or just don't know where to go, like they've forgotten the instructions on where to be. It can be very predictable, particularly with that lone player in the middle behind the attack. But then you also have players like Casemiro, Bruno, Rashford and Garnacho who can be very erratic in possession of the ball. I don't think a rigid systematic approach works when you have players like that in key attacking areas. Which brings you back to why he brought in Antony.
Now at Utd maybe he doesn't have players who are technically capable of playing at a higher intensity with the ball, the ball control and passing levels in 1 and 2 touch phases just isn't good enough. When Utd try to up the tempo, too often passes are to the players wrong side, behind them or up at their knee. Which is where it breaks down a lot in attacking areas and they get hit on the counter. And that's when you have the issue of that lack of pace in defence getting exposed.
Anyway, those are my thoughts, too slow and ponderous in the build up, which allows teams get setup and are harder to break down. And too sloppy and careless when trying to up the tempo, which leads to the lack of pace in midfield and defence getting exposed.