A few spring to mind.
I hated the ending of Niven and Pournelle's
Footfall. Not that it didn't make some sense to end it where it did. But it was just far too abrupt. I did feel cheated.
Then there was Peter F. Hamiltons
Reality Dysfunction. Which
really annoyed me because there is no indication on that thick fat tome that it's actually part one of trilogy. I was a struggling student at the time who couldn't afford extravagances like trilogies
(Well. I say trilogies. I think Hamilton just writes a couple of million words, then his publisher just chops it up into three equal size 'books'.) I eventually got round to finishing off the story, but I'm still in a strop with him and refuse to read anything else by him.
(Okay, it's really his publishers fault, I suppose, but the trilogy is okay, not brilliant, IMO. So I don't think I'm missing out much, in case someone chirps in. Still many other authors for me to read before I perhaps dip a tentative toe back into his work.)
But one author, who I do enjoy and love his ideas is Alastair Reynolds. Yet his Revelation space series...
..I think
Redemption Ark is by far the best of the bunch - the strongest work I've read of his, some great tight writing and ideas, yet there are two set pieces that could have been amazing, yet he glosses over them in a couple of lines. It's so disappointing. It feels as if these were editors cuts, maybe. Would be interested to know if that was the case.
But then the next book,
Absolution Gap....he kills off the story, and all the best characters from RA and goes off on a wander somewhere else. It has its moments and probably would have been better as a standalone in a different universe. It just feels that he didn't really know how to bring his story to some sort of conclusion. (Actually my guess is that he is a bit of pantser, given the work I've read. Generally his endings seem, to me, to peter out)