if your undervolt is crashing, its not stable.. undervolting can improve performance if u are hitting thermal limit. thats why its especially prevalent in laptops/sff. only this generation that nvidia and amd suddenly raise their power draw/heat output by so much and make undervolting more widespread. previously was vega and rdna which ppl got performance improvement from undervolting.
pascal is just crank gpu cores to max and play game
Definitely but some games can be isolated crashing like Apex Legends where vMin for GPU can throw you out to desktop. You can not crash for 99 games out there but you can crash for that 1 single game. The concept of stability is how much tolerance one have for issues like this. You can google and see how many people found out undervolting their GPUs even the older ones have caused crashes for Apex Legends.
Agree that TDP for the 3000 series is out of whack and it makes sense for undervolting if you are in a thermally limited case or concerned with power draw. But if you can be stable with undervolting for the games you play and prefer running less heat, noise and boost frequencies, then by all means. Efficient, yes. Optimal, depends on your priorities and limitations.
TLDR, Undervolting does not have any direct impact on your GPU untill there is some thermal throttling. Undervolting is good way to reduce GPU temp. and power consumption to maintain constant performance and should be preferred if your GPU temp. is hitting 75 C and you are thermally limited in your case and/or power supply.
RTX 3000 series is not exactly an efficient architecture given the power draw. Undervolting does not change this much despite running cooler. Power draw is also affected by things other than clock speed and voltage alone, but they definitely contribute.