Hot Take - The shows people seem to most strongly dislike the endings for often seem to be the ones where the ending resolves the story, as compared to ends it.
Star Trek TNG for instance was never building a story that required an ending per se, so while they connect some dots in "All Good Things" it isn't the conclusion of the story but just a really good last episode.
But for shows like Lost or Game of Thrones, the ending is expected to answer the core storyline that is baked into the shows DNA from the start ("Where are we and will we get home" and "Who will end up on the throne" respectively), which means everyone who invested time since the start and speculated over the outcome is comparing the ending to what they expected. I am actually really fascinated by the reactions to both conclusions.
Both really subverted audience expectations - for Lost, the idea the actual survivors of the Flight 815 really couldn't care less about whatever machinations was going on with the island, they just got stuck as pawns in some power play over the island when all they wanted was to live their lives and be happy - that perspective makes a lot of sense but went against the audience expectation of wanting to learn what the island was. In many ways the audience in fact was aligned with the "bad guys" from the show that cared more about the mystery of the island than the suffering that was caused by trying to understand or control it. So when the show came out that the important thing was the people and what happened to them and they didn't spend the finale laying everything out about how the island could jump around or what not, it wasn't what was expected.
GoT was similar in that so many fans picked what faction they wanted to sit on the Iron Throne, even though the show was trying to say that everyone who wanted it probably didn't deserve it. So while the show ended with the best thing for the people of Westeros - to have none of the houses win but instead move to a better form of government that wasn't based on crazy people lusting to have power because of some lineage (and the stupid throne was melted down) - it seems like many fans rejected that because they had already become invested in and accepting of the idea that the character they had already bent the knee to was deserving, or at least someone was going to win the Game at the end. Once again, many fans took the position that show was actively trying to debunk, that allegiance to someone and desire for power was not that great for peace and prosperity.
I know there are other objections to the final seasons of both Lost and GoT (I think Lost kind of got tripped up with the Sideways world and didn't really pull it off seamlessly, and I agree GoT kind of rushed through some things and maybe didn't get the motivations as clear as needed, but given the author can't seem to figure out how to resolve the story either I give the showrunners some slack on that)
One of the best endings of all time was The Shield, and even that riled up some fans who wanted to believe that Vic Mackey was a "real cop" and his actions were justified even when he killed other cops trying to do the right thing. But it pulled no punches on who Mackey was and what damage he caused.