I think it really depends on preparation. It's one of those dishes that varies wildly in how good it is based on a few simple things. Canned green beans? The casserole can't be saved, green beans are one of the worst foods to buy canned that a lot of people still do. Frozen green beans? You can make it good, but you've gotta do some doing. Fresh green beans? Even a mid recipe will be pretty dang good, but then you get it good and crispy and maybe you upgrade to a homemade cream of mushroom, and yeah it can steal the show. But people use canned green beans because nobody really likes it and it's just there to be a traditional vegetable option and so they don't know how little effort it needs to be drastically better.