I've noticed a nice little 'side effect' of Google Maps on CarPlay, before while Apple Maps was running on either the full screen or dashboard, any music playing would occasionally stutter. I thought it was an iOS or CarPlay bug, but I've just done an hour long drive using Google Maps (both fully open and on the dashboard) and there wasn't a single stutter. Perhaps just a coincidence and I'll have to monitor it, but maybe the bug was with Apple Maps after all, a side effect of turning off voice navigation maybe?
Also I'm pleased to report that if you just have Google Maps open and close Apple Maps on your phone, when you start the car the dashboard defaults straight to Google Maps, not Apple Maps, even if you have no route set. I was expecting the dashboard map to default to Apple Maps unless you set a route in Google Maps.