Unfortunately, an elevated number of groundings can indicate two different things: A cautious, proactive regulator and good safety culture, or a more reactive/lenient regulator and a plane so unsafe, they were finally forced to act.

Proxy measures about things passengers consider “bad” are probably more useful than groundings alone. Compute some score based on things like:

1) Number of passenger fatalities as a result of a mechanical failure of the aircraft.

2) Number of passenger items ejected from the aircraft during flight.