Here is your biggest risk:
This ROM is a package of modified software built by someone you don't know. He told you what he put in it, but you didn't disassemble it to check what's actually in it. There could be keyloggers, virii, trojans, etc etc etc etc in it. You would have no idea.
That's the worst case scenario. Very unlikely, especially this popular of a ROM.
The less severe issue, you're not going to get to install stock ROMs as they come available. If a serious security issue becomes apparent, you will not be able to install it without checking forums to make sure its safe to install over a hacked ROM. So there may be a slight delay in getting to use the latest security patches.
Otherwise, it's pretty darn safe. I wouldn't be installing it if it wasn't.