Yeah, the can opener is a really good move! sometimes (if your opponent is'nt very good) you can even make the guy submit.
Otherwise, the weakest spot in the guard is when they're doing a transmission to a submission/choke. for instance:
1.when he's tucking your arm under his leg for a triangle, just continue to push his leg over your head (if he's holding your hand, use your elbow/arm)and dodge the leg until your out, and then go for side mount.
2. do "the telephone" when he's going for an armbar