See, the beauty about this is it is a purely driver level tweak-- meaning it does absolutely nothing physical to the card itself. If you take your 6800 (16x6) out of your computer and plop it into a friend's, it will not have the pipes / vertex unlocked. That also explains the fact that you can just uninstall RivaTuner and the softmod will disappear. As stated previously, harming your card physically is probably the last thing this softmod will do... and it's very unlikely it will happen.
6800's are cards that may not be suited to be a 6800 Ultra for a few reasons: Damaged pipelines, damaged vertex shader, or unable to reach Ultra clock speeds. Instead of just pitching them, they mask 4 pipelines, 1 vertex shader, use 128MB of DDR1, and reduce the clock speeds down to 325/700. Now one or some (or even all) of these don't have to be damaged in order for them to downgrade. For example, you have a card that cannot hit 375MHz on the core... instead of throwing the card out, you drop the clock speed down to 325MHz, mask 4 pipelines (which are FULLY FUNCTIONAL), mask a vertex shader (which is ALSO FULLY FUNCTIONAL), and use 128mb of DDR1 RAM on it. That will, theorteically, enable a successful softmod.
You can also say the same about any of the other problems listed, except of course if the problem is related to the pipes; which in that case, you will not be able to softmod it for obvious reasons.