You must not play the class, or at least not both specs. The playstyle is VERY different. Mutilate is more single target oriented where sub takes a little more set up and is much more versatile thanks to dance. Sub can go on the offensive or defensive. Mutilate can't play both roles efficiently. If you're getting out pressured as mutilate, you're probably going to lose - sub can turn games around.
Though none of that matters at 90 because sub won't be viable. No crit as it is plus no more +crit chance to ambush = 0 damage, ever. And it's hard enough to garotte/ambush during dance as it is with the mobility problems (no move speed compensate with crippling nerf.)