And like I said - if you're doing AoE you're not doing the proper difficulty. Complaining that Shadow has garbage AoE and can't keep up in too-easy Mythic+ is like complaining that Shadow does no damage in a group of Mythic-geared raiders clearing a normal dungeon. Your entire thought process hinges on "We have to be AoEing down trash" when that isn't at all how it works in Mythic+ in Legion. If you have the availability to AoE things down then you need to bump the difficulty up.
If you're intentionally doing a lower difficulty than you're capable of then that's on you, not the class design.