Regarding risks, I ran into a problem with the custom MIUI ROM where I couldn't boot the phone anymore. I was scared there for a second (since I couldn't even get into the recovery mode) but I just found the current official Samsung firmware, flashed it and everything ran fine again. You can tell though that it has been modified, since each flash of a firmware increase an internal counter. So in theory there is always the risk of a bricked phone or voided warranty. But I don't know anybody who had this happen.