Xamarin is indeed the closest there is to the holy grail of cross-mobile development. I too had some success with it for a small project at first, but just like the theme of the blog post, the challenge is in the long-term maintenance being cost-effective, not just the initial development. Especially in the face of these adversarial platforms constantly changing things underfoot. For example in Android recently I've been forced to switch to the AndroidX library, and iOS has Dark Mode coming out. Will Xamarin support both of those platform changes cleanly? Maybe, but it's a risk.

I'd be happy to hear if the project you worked on has been cost-effective to maintain in the long term. Then at least someone has seen the mountain top!