I liked Bel Ami, why? For no other reason than it was a movie with no
explosions, car wrecks, stabbings, murders, chase scenes, which I won't watch, period. At least it tried to tell a story, and was a European period piece, at that, which I like.
Unfortunately your comment does pretty well cover all the movie has to recommend it.
I do think it's great to see a selfish protagonist and a lot of moral ambiguity. I also like that the movie didn't oversell its point, mostly left it open. (for example, was Duroy so ultimately unlikable as a demonstration that a nice face and an evening jacket is/was enough to get a young man anywhere in life? Because that's my draw on it, but it didn't seem terribly obvious) But that's about it, otherwise I found the movie to be rather bland. The houses and dresses outshone the actors, but that's all i wanted out of a random netflix pick.