>>57897886
Is it? Ubers is full of setup sweepers and Marshadow makes it dangerous to setup sweep.

>Immune to the most common priority move in the tier (Extreme Speed)
>Can live a hit with Focus Sash since ubers don't have multi-hit moves aside from Urshifu-R
>Has unresisted STAB
>Only support you need is something to remove Stealth Rocks
>The most used Pokemon in the tier runs Defog 99% of the time (Yveltal) and can U-turn into Marshadow if anything tries to set up on it

Marshadow then Close Combats/Spectral Thiefs your ass to death. And even if you don't have a setup sweeper, it can setup on its own with Bulk Up.