It is built on top of jQuery and extremely slow. They are very adamant on MVC with two-way databinding (the opposite of React or most other frontend libraries that have been around for almost 10 years). The backend request is geared toward OData (think graphql but table-oriented), and adding logic between the UI and the OData endpoint is hard.
SAPUI5 is hard to use even for a mediocre screen need. It aims to get the simple apps really simple to write (to lower development cost), but at that it kills the productivity of all the other apps.
Getting back to documentation, after your app is not just some components living in logical islands but have to interact with one another (think getting data from one place to drive another component) most of the time the documentation did not help with that, and I had to resort to debug the whole library internally to understand it.
Add all that to the fact that they don’t even care to support firefox for non-windows users bah
It is really understandable why no one but SAP uses this library. It was made for one use case by sap for sap and that’s it.