Well... low Piety = burn... Regardless of other things.

However, a lot of people seem to share the idea that Inquisitors are a bit too 'effective'.

Personally I have lost two HRE Emperors within a few turns of each other. One was in fact leading a Crusade. Obviously that sent me reeling in an attempt to send a general out to the Crusade in time. I managed it... But had the Emperor just managed to move a little bit further.