Any 3rd party solution will have some issues. Whenever you see mostly positive testimonials, you should be both suspicious and scared, because it means you will be the one to find those issues in production, and you won't find any help on the internet.

That was a long time the case for Xamarin, but if you look at their customer forums, reddit, stackoverflow etc you will find the negatives too nowadays.

I am not interested in .NET, so I would not personally use Xamarin anyway, but I know that with Qt for instance you have to be careful with mobile UIs - it's best used for custom stuff and not native-looking apps.