You'd need to know something about cinematic history to completely get it BUT...

Rating: 7/10 (28 January 2017)

... it really is a "Tale of the Christ" in allegory. Eddie Mannix is "the fixer" for a Hollywood movie studio, Capitol Pictures. His story is supposed to be in parallel with that of the Christ. This is not at all obvious. I had to watch it a couple of times - a couple of times in which I was actually trying to pick up all of the pieces of Hollywood history - to actually get it. Eddie is trying to quit smoking, and he is continually going to confession about his failures in this area. The devil is actually Lockheed, which offers Eddie a tempting offer to abandon his "flock" - the stars he looks after - in return for an easy job and financial security. He's got a family - wife and two kids - and television is on its way. What will he do? In the meantime Eddie's "flock", the stars of Capitol Pictures, are modelled after Hollywood history of the year in which this is set - 1951. Eddie is trying to deal with the unwed pregnancy of an Esther Williams clone, who curses, smokes (while pregnant), and is not at all sure who the father of her child is. Then there is the sudden disappearance of their biggest star, Baird Whitlock (George Clooney), without whom "Hail Caesar" cannot be completed. It makes fun of the singing cowboys, and what happens when you try to install one in a drawing room drama with a snooty directory, and even Carmen Miranda and Tilda Swinton as identical twins and competing gossip columnists - I am assuming she is supposed to be Hedda Hopper and Louella Parsons - are lampooned.
In this version of 1951, "the Commies" are real threats, not just boogeymen, and figure largely into the plot.
If the Cohens made a mistake here it would be depending upon the audience knowing about old Hollywood and getting the allegory of Eddie Mannix as the Christ. You could watch and enjoy "Fargo" and know nothing about Minnesota, you could watch and enjoy "Blood Simple" and "Raising Arizona" and know nothing about the American southwest, but it might be tough going to watch this and get all of the jokes and plot points and know nothing of the history of Old Hollywood.