I really like the Kivy project's goals and the demos I saw were super fast and nice. But when I had a look at actually building something with it I discovered that the default UI is completely raster based, which is just a weird deal breaker.
If there was one thing I could wish to be revamped about Kivy it's the default widgets. There's a core of good ones, especially those that are most commonly used, but quite a few make bad design decisions like an over-reliance on images for styling. This is often much easier to work around than is probably obvious and maybe that's part of the problem, more experienced developers likely habitually sidestep the parts that don't work well. But on the other hand, it's quite a bit of work to revamp them (including documentation and dealing somehow with breaking changes it would introduce), unfortunately that has kept us from seriously addressing it so far.