Cons: I attempted to use the Core Unlocker in the BIOS. The BIOS brought it right up and recognized it as a Phenom X4 B55. Unfortunately Windows7 64-bit and Ubuntu 11.10 Ocelot 64-bit wouldn't boot. W7 BSOD'd and Ubuntu 11.10 simply stopped loading.
It seems that W7 was waiting for the 4th core to send back a signal to enumerate and it wasn't able to. Someone of Geek Status might know more, but this Nerd will simply use the 3 cores he paid for and not look a gift-horse in the mouth. This isn't a con for me, but it could be if this is what you expected from the core.
Emphasis mine.
So, you're worried because a system that will not load Win7 could also not load Ubuntu. The common factor here is trying to get a 4th core for free. If you don't unlock the chip it will be fine.
It seems the week point here is the unlock tool not verifying that it's done its job correctly. I know AMD have a good reputation for OC's and unlocking but ultimately they bin chips based on yield, you can't reasonably expect a 100% guarantee on turning on features AMD deamed the chip not to have.