I have a daughter who says that she is still mad at that show for that ending. I always thought that it was because it confirmed, what I had thought all along, that there were religious overtones to the show, and they had to meet up before they continued on to the great unknown. She hated it, I always felt it was just right. Hubby and I just binge watched Lost a few months ago; and, he was fine with the ending, too. So, I think the people who hated that ending, are probably people who don't like religion, or even a hint of it, especially Christianity.
Most of the ones I know that complain about the complaint is the island is never explained. They want to know how it works and why it works that way. My counter that is always one word: midichlorians. When Lucas explained The Force he made it boring, it went from being mysterious and mystical to parasites. The only explanation I need for Lost is “magic island”, anything else would almost certainly wind up being midichlorians.