Short answer, Flutter is much better than Xamarin and Qt and it is gaining momentum. It is easier to code in dart since it's the only one language for everything you need (UI, Logic) to build apps on multiple platforms. I have built a geofencing app using google maps API for flutter (google_maps_flutter) and using layers I created with ArcMap. The only drawbacks from my approach is that the app lag a bit as it is trying to load so many polygons from so many layers(I guess in the future this will not be a problem as the platform mature and phone hardware gets better). The only challenge that I am facing now is google_maps_flutter still does not support geometric operations on polygons and polylines which prevents me from creating a mobile app capable of advanced geoprocessing. When google_maps_flutter gets the capability of geometric operations I will find less need of use for ESRI Mobile API for my mobile dev endeavour, which would be a shame really. I hope ESRI will jump on Flutter train soon.
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