I had a pair of 5* Inquisitors doing over the Pope every year for more than a decade, with 0% chance every time. They turned him into the Born Again Atheist but he died of old age before they could fry him. This, seemingly, in spite of high zeal rating in Papal States, all the while.

I have noticed that Faction leaders get similar zero readings, regardless of piety rating or the zeal of the province they are in. Heirs get a very low percentage and high-star generals are similarly well protected.

I think it's similar to assassin success ratings, where the faction leader and heirs get a level of automatic protection ('well guarded') and high-star generals have a tendency to accumulate a personal valour level which the assassin probably cannot match. (Don't just compare a general's command stars with the assassin's valour stars, look at his unit's valour as well. If that's their average valour, then have a guess at the general's personal valour score and factor that into the equation too. Don't forget that, for each pair of command stars, he gets +1 valour boost on top).

For inquisitions in general, the best targets for newbie Inquisitors are, as was stated previously, your own low-piety unit leaders - no fear of him getting assassinated if your home territories are properly secured against intruders.

Best odds are given on zero-piety unit leaders. It's then 1,2,4,8,16 victims to get them to five stars but 4 'home' victims and three stars should give them good survival odds abroad and taking out some 3 or 4 star enemy generals will advance them up the valour ladder that bit faster because a 4-star general is 'worth' the equivalent of 8 0-star victims.

Or that's my theory of how it works, anyway.

When the enemy stack leader has zero piety, the success rating is exactly the same as the level of zeal in the province. Each piety cross reduces the success percentage by quite a significant amount, to the point where even 90% zeal is of no assistance against a 9-piety general, with success percentage in single figures, as low as for an heir.

If necessary, you can allow the Inquisitor to preach for one or two years in the enemy province, to ramp up the zeal rating before beginning to carry out attacks on local generals but try not to leave them preaching for too long. As soon as it gets 'too high' and they begin burning the local population, the zeal takes a sudden drop, likely to below where it was when you started. This is all too easy to do when you're preoccupied with military aspects of your campaign and your inquisitor is off in some part of the map away from what's holding your attention.