I like the Van Helsing analogy.
Something also of note is Kincaid was impressed by Harry's ability and power in Blood Rites, where even Harry admits he didn't let loose. But Kincaid says, he'd never seen a wizard let loose before.
So therefore we know that Kincaid has never actually fully dueled Ebenezar despite knowing he was the Blackstaff, and whatever went down between them.
I also suspect Kincaid doesn't just limit himself to supernatural contracts, but does some mortal ones as well.
Goodman Grey did take out a mortal trafficking ring
Honestly the one thing about Kincaid that annoys me the most is that if Harry had called in his favour for Kincaid to back him up at Chitzen Itza we could of seen Kincaid all out on the battlefield, which I hope we see in Peace Talks or Battle Ground.