I've just tried using the new RHI backend with a non-trivial application on macOS Metal. Very impressive!
However It looks like transparent toplevel windows are not yet supported. This also used to be a problem in QtQuick 2 early days (
The following example uses a transparent window background with QSG_RHI=0 but a black background with QSG_RHI=1.
Setting QQuickWindow::setDefaultAlphaBuffer(true) early in main.cpp does not make any difference.