So I just ran an adventure with two cavaliers as NPCs. I found on a whole they were pretty good as combat tanks although they were outperformed by other combat classes when they were on foot.
I used them both as allies for the PCs for a couple of battles, and then had one dominated into becoming a villain so that I could see how they performed as an NPC combat encounter.
One thing I did want to mention that flummoxed me a bit using them as NPCs was that it is hard to keep track of oaths. As a player character I'd imagine that making and using oaths would be imperative... rather like memorizing spells, you don't want the DM to forget.
But as an NPC... there are so many things to keep track of, especially with a large cast of characters. It was really easy to forget about or lose track of that free floating and mutating bonus. It was one of those numbers that kept "disappearing" on me. There is no really easy way to keep track of it unless you rule that the NPC cavalier has one oath that he just constantly re-swears.
I'd say that this class on a whole is more workable as a PC than an NPC. I'd probably use a paladin or a fighter with a lot of flavour instead for my villains, the same way it is much easier to use a sorcerer NPC than wizard.
