It must be the four titles in a row, because anyone who has actually seen his Ajax knows he has bored the fans to death, and they know how to appreciate possession football and they are quite forgiving when there's a bit of dwelling because they are absolutely not forgiving of untidy passing. The majority wants him to go, bad football, titles because of failing competition, terrible cup results, teribble European results, beaten and humiliated by lot lesser clubs, terrible signings.
He tries to imitate LvG's style, but the only part he understands is the possession. Doing something with it, like beeing defensively strong and producing free flowing football like LvG generally does (allthoug he has often struggled here with that) is compleletly beyond him as the last 5 years have shown. And that's with Ajax, those players practically are bred for that. It's not new to them like United players, they don't have to do it at pace and under the physical pressure of the PL. He has unlearned them how to attack, it generally is worse than LvG's United at it's most boring. Besides the boredom, also on quality and perfomance he's nowhere near Van Gaal, for some reason Van Gaal is not universally loved at Ajax, but you won't find an Ajax fan who think De Boer is even close as good a manager as Van Gaal. The difference is much bigger than between Moyes nd SAF, some even hope he gets sacked at United so he can take over Ajax, with these players, he would cruise to titles and be a strong in Europe again.
So I know how his style will be recieved by the fans, not even mentioning performances, no way there's enough patience at any club in England for his style to be well recieved.