I think Moyes had the hardest job at Manchester United by far out the managers we've had since Ferguson.
He was taking over from a manager who had been in charge for 26 years and had loads of success. The problem with having a manager for such a long time is that the structure became outdated. Manchester United were still run like a club from the 90s, which was mainly built around the relationship between Ferguson and Gill. They both left in the same season, resulting in Moyes having to deal with a new and inexperienced CEO. Ed Woodward, clearly, did not know what he was doing. Moyes did not have experience at a top club, and I think it is vital for there to be experienced people in place if you go for a manager like this (and even when you don't). It was like taking two trainees and thrusting them in the position of two very experienced people.
This resulted in a terrible window. Perhaps Moyes overestimated what Manutd could do in the transfer window, like Van Gaal said he did, but there was no experienced person in place to advise him about this. Moyes had the right standard of player in mind: Fabregas, Kroos, Bale etc. but he overestimated our capability, especially with Woodward in charge. I know we ended up with Fellaini, but that was, unquestionably, out of sheer desperation for a signing. There is no other reason to sign him for the increased amount if Moyes actually wanted him at the start of the window. I feel it was more a 'take it or leave it' situation at that point: either he had Fellaini or he had nobody.
One issue that is underestimated is this: a lot of the players had played under Ferguson for a very long time, and had become accustomed to him being manager and the way things were done. By the time Moyes became manager, most of these players were on the way to retirement. Most of them were gone within one or two seasons. I do think this probably played on the player's minds: were they so bothered to accept a new manager when their careers were on the way out? They may not have meant to do so, but this can be a subconscious thing when a deeply-set routine is involved.
The team didn't have any promising young players. It was old, and there was no promise in that team, really. It was stale. Yes, they won the league in 2012-13, but most of that was to do with Van Persie and Carrick. The team itself wasn't particularly great that season. Van Persie, in particular, makes the performances seem better than they were at that time. That is the main reason why I think people remember it more fondly, and due to it being Ferguson's last season. The combination of this and the terrible transfer window was an absolute disaster.
In the end, I do think Moyes made mistakes, but I do feel people underestimate the monstrous task he had when taking over the role. People continually point to the win the season before, but forget that the performances had not been great for two seasons. The team was on the wane with no young players ready to take the older player's places. It was actually a bit of a mess that was being held together by Ferguson. Once he let go, the team and the players really fell to pieces. I feel like Moyes was actually lumbered with a squad that was about to expire.