We are a "on property" family so this is slanted because of Uni's policy of giving UEP to onsite families.
We love the UEP from Uni. We like to ride the Mummy, MIB, Hulk, Despicable Me, Transformers, Rip Ride Rockit and Spiderman multiple times (and back to back if we want). UEP allows us to do this which would otherwise be impossible without the waiting. The rides mentioned above all had waits of 45 min plus at the time of year we went. It is a time saver, money aside. As far as dealing with no EP for Gringotts, we took advantage of early entry and got our fill before the masses showed up. And Forbidden Journey, we just went there first and had minimal wait. No different than we would do for certain rides at Disney, big difference is it was only one attraction.
We love Disney, it is the first for our family but I'm not enamored with the FP+ system. I like to plan but securing FP so far in advance is a little much. And this idea of being able to get whatever you want the day you get to the park via kiosk was not our experience. We were able most days to get an extra FP and even two on a couple of days but I had to purposely book our first 3 for morning times everyday and then had to "settle" for whatever time (and whatever attraction) was left for our additional FP. It's fine and all but not near as good as Uni.
All of the above hasn't stopped us from going to Disney but I do appreciate what Uni has to offer. For our family, the two are just different experiences. We are happy they are both available.