I never did understand that hate for the ending of 3.
Well for one, they promised that your decisions would tie into what happens at the end, especially character wise, and that didn't really pan out, not even with the extended cut DLC. Plus EC just showed 3 color coded endings, they were all too similar. It wasn't true branching paths, just 3 cutscenes that were nearly identical save for the color coding.
I'm not saying that was a deal breaker for me, because I still think ME3 was best for the more polished combat, dark atmosphere, creatures, and DLC, but I can definitely see why many were very put off by it.