How to uninstall a system app using adb uninstall command (NOT remove via rm or any other way)
I succeded adding the -k and --user options to the pm command, e.g.:

pm uninstall -k --user 0 com.android.service


The command was issued as root, no need to remount /system as rw. Android on the phone is 6.0 and the package is a system app. Without the extra option I get the error:

Failure [DELETE_FAILED_INTERNAL_ERROR]