I think you answered your own question. A lot of the hate comes from all of the kinks. Folks wasting hours and hours because
MDE loses their FPs in the etherworld or is way too slow and/or regularly crashes on their smartphone to the point of it being non-functional.
If they get all of the kinks worked out then some of the hate will subside.
Other valid reasons people can and do "hate" FP+ are that:
- they do not like to plan ride times 60 days (or any days) in advance. It reduces spontaneity for those who like spontaneity, and it reduces flexibility in going to a different park on a given day for those who like flexibility.
- apparently there are now substantial lines for rides that never used to have substantial lines - such as SpaceShip Earth and Figment JII.
I have not used it yet and even though I am more of planner type,
I dislike the idea of choosing ride times in advance. However, I do like the idea of not having to rush around getting FPs. I do like the idea of being able to show up late to a park and still being able to get on top rides. I dislike the idea of teired rides where I have to choose between Soarin and TT at Epcot and cannot get both.
Those are some of the valid reasons for the "hate". Then there are folks who just plain dislike change. That is another discussion.