Mutilate is the most pressed button on ST on Assa rogue, yet it does close to no damage. On a current raiding parse, the average envenom hits for 3m, while you have the average mutilate hitting for 170k. For comparison, each garrote ticks for over 200k, which is more than your direct mutilate hit.
I know it’s more of a means to an end (combo points), and it also procs poisons etc, but I do not think having your generators do 0 damage feels any good.
If you look at enhancement shaman, even though they were one of the most powerful s1 specs in TWW, it felt horrible pressing stormstrike outside of ascendance because it did no damage. In s2 with the damage redistribution from spenders into builders, the spec feels much better to play (even though it might perform worse).
As far as assassination rogue goes, I think they are due for a damage redistribution, there is too much damage in random procs & dots.
There is too much damage in Envenom - it does from 30 to 35% total damage of an Assa Rogue, barely less than all DoTs (Rupture, Garrote, DP, DM, KB) combined - all of them in total barely edge out Envenom at ~36-40% total damage. Dots are doing fine, Assa has been a DoT spec since at least WotLK.
I do agree that Mutilate just doesn’t feel good to press, you basically only care about it giving you combo points. But if anything, it’d be best to focus on how much damage is shunted into Envenom and maybe redistribute some of that into Mutilate.
Also, Rogue is due for another rework, IMO. Currently, neither Assa nor Outlaw are well-designed - Assa is carried by its good numbers in high keys (feels terrible before that though) and raid, poor Outlaw is just bad all around.