Can opener is fine if you are not rolling with a guy with a really good guard who's well aware of the can opener. If you are rolling with a pretty good grappler you're likely going to end up in a lot of trouble trying a can opener.
Not to mention most lower belt rank classes in competition do not allow neck cranks. The can opener is a neck crank. You can use it to pass the guard, but there is a fine line in most competitions. I have seen guys tap because someone was using the can opener to pass and the guy using the can opener was DQ'd for using it and the other guy tapping.