I had no problems with Looper's ending, nor Snowpiercer, Under the Skin, Moon, Source Code, Children of Men...
but yeah, very often Sci-Fi shits the bed near the end, too many turn into action movies or something for the last act. Hell, more often then not most movies don't really stick the landing. I'm a big Nolan fan, but even I'd say that most of his films didn't end all that well (exceptions made for Memento, The Prestige, and Following).