I think its more a SP grievance then a warlock one. While they suffer the same it is spammable for them and ranged so they find more windows naturally to get value out of it.
Having a 30 second CD on it as your only cross with blanket but having to deal with sanc which prevents the blanket cross, and war shout which is basically a sanc as well is tilt.
Tremor obviously as well.
Most lobbies I think im in are undead healer often enough with ele/enhance/arms/fury/ret present one or multiple or all.
Then if you get a dragon healer with them they can just wall and be immune to blanket for 12s. Finding actual windows to stick a full CC to get momentum feels impossible sometimes. Even after the legwork of trying to catch a fear to begin with.
maybe its just a too many fear out complaint though.