The other thing that was uniquely terrible about FP+ was tiering.
The three WDW second gates have so few attractions that naturally everyone wanted to book the same thing. So they created tiering, which basically meant that for Epcot and DHS, and later Animal Kingdom when Pandora was added, you could only book one popular attraction in your three. So you couldn't book Soarin' AND Test Track, you had to choose. You couldn't book RNRC OR TSM, you had to pick one. You couldn't do both Avatar attractions.
MK you at least got to do what you wanted, but that didn't necessarily mean you were home free either. In ALL the parks there were some attractions that were very, VERY hard to get, if you weren't staying at a Disney resort, because Disney resort guests could book 60 days out vs. 30 days out for everyone else, including APs. So it was basically impossible to get Seven Dwarfs Mine Train, Frozen, or Flight of Passage unless you were giving Disney money to stay at one of their hotels.
Believe it or not, people miss this.