Higher C on the battery than what is needed is better for the battery and esc. If you have too low of C, the esc tries pulling amps it can't get, then it runs hot and the battery fails. Usually the battery gets destroyed in the process by puffing or bursting into flames. In most cases, if the battery says 50C on it, it's likely 30C constant (regardless what is written on it), so for most cases, go with at least a battery rated for 50C if you have a system that requires 30C.