I think most people are not "bitching about an unfinished product" but moreso being dissapointed/annoyed at getting something rather different than what was promised (A game that resembles 5e closely).
If you order a medium-rare steak and you get a well done one, for the same price, you'd be rather annoyed, I figure.
An unfinished game that's mechanics will be in flux until it's out of EA. If I went to a restaurant where I could pay for all you can eat steak while they train new cooks or try out new things on the menu I would expect certain things not to be the way I want them. If you have in your mind what you wanted and won't be happy unless it's exactly that thing, you should have waited until the game was out of EA and reviewed to make sure it was that thing.
You don't get the point. If there was any feedback/indication from Larian that they indeed intend to make this an actual D&D 5e game (to a reasonable extent, of course) and the rules were just not fully implemented yet, or at least that they realize e.g. that their handling of advantage/disadvantage (not specific bugs, those will obviously be fixed along the way) in combat is currently broken, nobody would be complaining - OK, that's a lie, but no one reasonable would. The opposite is true: Larian explicitly mentioned that they tried to implemented the rules, but "it was not fun" (whatever that meant). So instead we get this weird hybrid that really doesn't work well and is insanely abusable once you figure things out. I DO like BG3 so far, but once you figure out the mechanics, the combat kinda sucks. At least once you figure out that you can basically get advantage for almost every attack and that every char should dual-wield to get attack as a bonus action, you can get through the encounters quickly.
EA is meant for exactly this kind of feedback.