It's an overreaction to hate what you know you have enjoyed playing up to the ending, although there is a temptation to feel that the entire experience has been soured by the final disappointing moments, not least because they are so fresh in your mind. We need to shelve the overreaction, take a walk, sit down and think over the game as a whole.
The ending has greater importance in some games than others. The ending of a trilogy, for instance, has roughly three times the importance (assuming that all 3 parts of the trilogy were building up the same story rather than being separate). So sadly, if it fails to deliver satisfactory closure, the negative player reaction is likely to be about three times as bad. It is not only the ending, it is the ending of endings, so to speak.
The importance of the ending is also affected by previous installations in the franchise. Borderlands didn't have an exciting or noteworthy ending, but that was hardly what the game was about. It isn't fair to hate something entirely because of a few bad moments, even if replaying the game is rendered considerably less enjoyable by the knowledge of disappointment to come.