I have over 20 years experience with Visual Studio (VB, VB.Net) desktop and Windows Mobile and over 6 years experience with Xamarin C# Android/Forms. Commercial applications for major retailers.
Windows Mobile is dead so no use talking about it. I converted over 20,000 lines of VB.Net Windows Mobile code to C# Android/Xamarin. The layouts are fixed and locked to portrait, so the WYSIWYG editor was useful. Created another Android/Xamarin app that required separate layouts for portrait/landscape and phone/tablet (4 total). Had to hard code AXML layouts to accomplish this (WYSIWYG useless). Converted the app to B4X/XUI and have one Shared Class with Visual Designer script that handles portrait/landscape in each layout (copy/paste). In addition, the app now works with Android, iOS and Java with minimal conditional compilation!
Double clicking on a visual component to create code is pretty useless as it only creates a default event. I prefer B4X right click technique for definitions and events since I use several events for many visual components (FocusChanged, EnterPressed, TextChanged). With Xamarin Android I had to hand code the definitions and FindByViewId declarations. B4X makes that process so much easier (e.g. copy and paste layouts and right click) . To get cross platform with Xamarin you must use Forms and text edit XAML code. Far more difficult than B4X Common Classes and copy/paste layouts (anchors, autoscale, etc). To handle orientation change in Xamarin Forms was very difficult as the only example on the internet had a bug in it that no one discovered for several years (I posted the correction to GitHub).
My only issues with B4X are not being able to develop for Android Wear because library isn't AndroidX and not being able to replicate an inclinometer I did with Xamarin Android (still trying).
Absolutely love the Hosted Build Server for iOS. I have a MacBook, but a real pain to fire it up to test and keep XCODE and Xamarin in sync. The B4XComboBox is so much better than Xamarin Forms which uses the ugly iOS Picker (slot machine spinner).
I will be using B4X for all future development. Thanks B4X community.