I really just hate cannons. Not because they're too strong, they seem priced pretty fair for what they can do, but their rules are just non-nonsensical in 8th. BS factoring into their aiming is essential for them to make sense.
The challenge rules do definitely need work. I think there needs to be some sort of system where a champion challenging a terror causing gigantic monster that will SURELY kill him, should need to pass at the very least a leadership test. Overkill on dead champion body needs to happen too, but I've made a lot of posts on that before just because it's such a given.