Fortunately, I think the "different results from different phones" issue you mention is mostly in the past. Most iPhone users update to the latest version of iOS very quickly, and while that's definitely more of an issue on Android, changes that have been around for years now have helped. For example, the web browsers/webview are decoupled from the Android version, so users can update those without having to update their Android version.
With Capacitor, we purposely use the stock WebView from the system, which is easier to maintain and are the ones that get updated/maintained by vendors regularly. Most users use these without even knowing. As for custom ROMs - well, that's just a very tiny slice of the market and if you customize beyond what is standard, you're risking issues across the board, not just with WebViews.
Fortunately, I think the "different results from different phones" issue you mention is mostly in the past. Most iPhone users update to the latest version of iOS very quickly, and while that's definitely more of an issue on Android, changes that have been around for years now have helped. For example, the web browsers/webview are decoupled from the Android version, so users can update those without having to update their Android version.
With Capacitor, we purposely use the stock WebView from the system, which is easier to maintain and are the ones that get updated/maintained by vendors regularly. Most users use these without even knowing. As for custom ROMs - well, that's just a very tiny slice of the market and if you customize beyond what is standard, you're risking issues across the board, not just with WebViews.
For reference, Capacitor's dependencies are here.