I tested CSS viewport units in 10 mobile browsers.
1. The units are not correctly implemented in the four smaller iOS browsers. (Only Safari and Chrome behave correctly.)
2. The virtual keyboard resizes the layout viewport in Firefox on both platforms, Brave on iOS, and Samsung on Android.
Test page: https://output.jsbin.com/jexifog/quiet
https://viewports.fyi reminded me to check in 3D touch preview on iOS.
So in the absence of dynamic UI, all units are equivalent, like in home screen web apps and desktop browsers.