It's not a question of faith, I know the philosophy is sound. It has worked everywhere and led to serious overachievements, it hasn't worked everywhere all the time, but nowhere there's been an 20 month period without it baring fruit. The question is why it doesn't work at United. The answer probably lies in what the philosophy is and what it isn't.
It started and developped as a set of methods to make the squad of a club perform better with more team spirit and better organized attacking football. But not just any club, it was only implemented at clubs with players that were technically better and smarter than most of the competing clubs. It was never designed for or tested on relegation football or typical midtable football. The starting point is that the players are better footballers than the opponent, so they will have to play much better than the opponent. The opponent having the technically better players is the exception, and this exception can be handled by better playing as a team. That always was the case, at Ajax, at Barca, at AZ, at Bayern and at the Dutch national team. They might not have been particularly good teams in everyway when he arrived, but they were always technically superior to most in passing, first touch, close control etc, just the basics of the possession game.
So probably the reason that it doesn't work here is because the players aren't technically superior nor smarter, or the difference with most opponents is too small. They have to move the ball around faster, but they don't have the particular talent to move the ball around fast. If they play too slow it doesn't open up defences, and if they move it around fast enough they lose the ball under pressure. With a few players aging quickly, some other weak spots and the impossibility/lack of success in signing technically excellent players, implementing the philosophy on the given squad has become an experiment, a first. Does it also work when the players have hardly any technical superiority over most of the opponents and aren't the smartest either?
Appearantly not. It seemed to work at many occasions, but the consistency isn't there, not from match to match, but also not within one match. If they played all matches like they did against Chelsea for the largest part of that match, they would win the title. But for it to really work they need the consistency and that takes more quality than just a good half or two good matches in a row. If I look at the passing, first touches and vision today of Rooney, Smalling, Carrick, Schneiderlin, Lingard and even Mata and you'd turn the question around and ask what would be the kind of football that will make those players shine most, it's just not possession football. With a Rooney and Van Persie as good as a couple of years ago, Di Maria, an in form Herrera, Shaw, Schweinsteiger, the answer could be different.
I wonder if LvG had known beforehand what the technical skills of the players were he would end up with against Sunderland, he would have chosen this style, or even this club.