They are just AC units in reverse. The biggest effect humidity is going to have is on how much condensation is going to form on the exterior radiator. That’ll form frost that’ll have to be melted in a defrosting cycle. That’ll decrease performance and efficacy. Low humidity should keep that to a minimum.
From description of the Great Old One pact:
Patrons can have whatever motivation or lack of motivation the DM wants. I personally like there being some kind of RPing effect. Maybe some task you have to complete, punishment for acting against your patron, slowly eroding sanity. But none of that is necessary.