You know, I've never played any Mass Effect game, but I have a lot of friends who do, and have talked about this at length.
I don't think anyone's upset that unhappy endings are possible. They're upset that only unhappy endings are possible. For that matter, they're upset that only one ending is really possible--every possible ending is a different flavor of the same scenario. No amount of effort you put in over the course of three games matters, only one decision you make really affects the outcome, and even then no matter what you pick, it's basically the same thing as if you had picked something else.
It's not what people were promised, and I think people have a right to be upset over that. If they had said "yeah, every ending is basically the same, and all are pretty much a 'fuck you' to the player", no one would have room to say anything. Instead, they were promised meaningfully different endings and a bunch of other stuff that simply wasn't delivered.