Desolation needs a lot of support to be a useful model. It's a great attrition piece if it can get stuck in before losing all its wounds. Between the built in heal on attacks, servant of dark powers, and the entropy healing off of levi, it can drain tank quite effectively. Its main weaknesses are a lack of bonus action and low defense, but the built in trigger and htw can still make it trade favorably.
I've had some success with bringing von Schill along to support it. Getting a leap in combination with the Herald move on SodP means it can get quite far up the board, resolving a lot of the issues the model otherwise can have. Without the positioning support to allow it to quickly start getting value from flurry and entropy, it definitely can struggle.