I don't know about Apple having a clause blocking other apis, but I do remember seeing something about blocking interpreted code which includes .Net and Java. Apple requires the software on iOS devices to use natively compiled code. For cross-platform there are two common ways around this that I know of. Make an app that just has a browser control and make the app a rich website, which ironically is much slower than compiled code. The other involves making an app using the Xamarin toolset. I remember reading Xamarin has to do some special stuff to make sure generics work when C# is compiled to an iOS app because in .Net generics are handled at run-time.

I don't know what kinds of apps you work on, but if you're making something to run on Windows then going forward a UWP is generally a good idea. As long as you're not stopped by some technological reason then I really have to ask why not? Microsoft has some pretty good development tools in Visual Studio for creating those apps and with them you can still make the app run on Desktop/Laptop/Tablet/Phone/Etc with support for handling the different screen sizes. I like telling people to at least look into it from a cross-platform perspective. If you're using the same backing logic but different UI code, or similar UI code, then the next logical step is to look into using the Xamarin tools. Yes, those tools looks expensive, but now you're saving money by not hiring more people to work on separate apps. It's all one team working on one app with one set of tools. I feel like I should say that I do work for Microsoft, but these are my own thoughts and my work in the customer support area does not does not include anything related to this. I just play with this for fun on my own time.