Hm, as no one has replied yet I'll give my not very insightful opinion.
First of all I like kivy. It is a mature platform with good documentation. Although I haven't used its mobile capabilities yet, there are plenty of apps to proove them. The problem is that kivy does not conform to the term native app in a strict sense as it will run on an interpreter. The other mentioned environment I can say something about is xamarin Forms. It is obviously very mature and the only environment I would trust with critical tasks. But it is not Python 😭.
In the end it is impossible to give you a meaningful answer, as what will work and what not, will highly depend on what you are trying to do. I would pick kivy if I wanted to target multiple platforms and pythonista if I only watend to target iOS. If performance is an issue in any constellation everything except the xamarin platform seems to be a really bad idea.