So? Do you want Knights to be unbeatable? They clearly aren't, as of the books. Badass, yes, but also very human and vulnerable. They can kill just about anything, which is as it should be, but a Knight never takes a hit from something superhuman and shrugs it off, and as for mental attacks, well, their Conviction can handle most of those they should be resisting, and with Superb Weapons or so they can hit most stuff (even Inhumanly fast stuff).