I used xamarin a few years ago for a moderately sized iOS app. It got me about 95% there. That last few percent was brutal, the app kept crashing after coming back from the contact chooser. Couldn't find a way around it, even with latest xamarin version, hot fixes. Was very unfortunate. I'm pretty skeptical of frameworks that abstract away a whole system. Hopefully, xamarin is a lot more stable and powerful these days.