This is a reasonably entertaining, but oddly paced and very traditional film that dramatizes the efforts the US went to in order to rescue the art that the Nazis stole in World War II. As far as I can tell, it is very, very, very loosely based on the true story.
The first, and biggest problem with the film is its lack of historical accuracy. The major characters’ names have been changed but that’s far from the biggest detail that is altered. What the unit recaptured, how they did it and where they did it (and, really, it was a series of units) is altered for dramatic effect. As is the size of the unit, which is rendered as a few people in the film but which was quite large in real life (as it would have to be). I get the idea of having a bunch of old soldiers wandering around Europe. I see the appeal in that. But maybe just write a fictional film about it.
The other major problem, as I see it, is that Clooney is quite a traditional Hollywood filmmaker, particularly here. He seems to want to tell a story that amuses us, but also makes us care about the cause of WWII in general and the Monuments Men in particular, and that also emphasizes the gravity of what the Nazis were doing. Like an old Hollywood film, Clooney’s movie wants to have it all ways and wants to be all thing to all people, meaning that tonally it is extremely confused, varying from mild humour much of the time (with a “bunch of misfits on an adventure” vibe), to moments of serious drama and lots of speechifying about the value of art. It doesn’t work.
I don’t understand why the real story couldn’t have been told and, I also don’t understand why you wouldn’t commit to either treating it like a comedy or playing it entirely serious.