They usually undervolt/clock the GPU, but not the memory. VRAM temps can be an issue and only some miners will try to address that properly with copper shims, etc.
The 3090 is notoriously bad for mining because it has memory chips on both sides of the board, leading to more heating in one spot and poor cooling for the 'back side' chip. Other cards tolerate it much better, but it can be an issue.
So while overall mining cards should be good, there are some specific issues to watch out for. A failed memory chip is not an easy fix, requiring a reball/replacement of that chip.
The 3090 is notoriously bad for mining because it has memory chips on both sides of the board, leading to more heating in one spot and poor cooling for the 'back side' chip. Other cards tolerate it much better, but it can be an issue.
So while overall mining cards should be good, there are some specific issues to watch out for. A failed memory chip is not an easy fix, requiring a reball/replacement of that chip.