I have used crippled servos as parts mines for ages, but, a few days ago the magic smoke came out of one. I tried the motor with a battery and it rotated slowly. Measuring the resistance, it was 0.1 ohms. Normally I would expect to see about 10+ ohms. Just goes to show that one problem can create more.
I have seen a few adverts on fleabay for ESCs that say they have a 6 volt BEC. Without buying one and testing there is no telling whether the 6 is a fact or casual typing by the seller. From what I have seen of BEC RXs, the BEC lives between the "batt" connection and the rest of the system. Any higher voltage being applied via the other connections will not be regulated, and will get fed to everything. If whatever is plugged in there is designed with a 5 volt max in mind, things will fail. If they are getting more power and being caused to work harder than intended, again, they will fail.