(For the record, Bethesda game bugs are a wholly different entity than most games bugs because they're almost uniformly hilarious, such as the backward-flying dragons, home-run hitting giants, and chickens that report crime.)
And why should the story ending of a video game affect the rating said game is given?
Bioware actually lost me on ME2, so I haven't played ME3, but the ending to ME3 is almost universally reviled because it renders every choice you've made up to that point pretty much pointless. Bear in mind that "your choices make a difference" is one of the biggest selling features of the franchise. Also, most of the ME series tapped into a Lovecraftian "extra-galactic evil" motif that took a lot of inspiration from the Cthulu mythos. The ending of ME3 had more in common with a bad sci-fi anime.
The problem with the critics in this case is that hardly any of them mentioned the ending as a bone of contention, but when the public got their hands on it, there was universal nerd-rage the likes of which has rarely ever been seen.