The biggest downside I found with kivy is the amount of time it takes to launch the app as it has to set up the python environment et al. If that's improved with newer versions of kivy I might go back to see what it can do.
If you're interested, you could try perhaps FlatJewels from the play store. It's a simple game with a fairly quick loading time (except the first run, which is always slower). I don't know if the author put effort into specifically optimising its start time, but it's probably fairly representative of what's currently normal.
It's certainly another downside of course, but to be clear to anyone reading the apks are much less than 25MB. And 25MB actually sounds like more than I'd expect even uncompressed, but I haven't looked at it much.