>>31707751Decidueye plays a different role. Its most common set after the Baton Pass ban is a Swords Dance trapping set with Decidium Z.
As for Tsareena, it also gets Rapid Spin, but it has recovery, so it outclasses Dhelmise.
Dhelmise simply has no niche in UU. Latias, Starmie, Togekiss, Mantine, Empoleon, etc. are all better hazard removers.