Us' higher hp is about the only merit I can think of. It would be a better damage soak than the imp. But given all the other advantages the imp brings to the table, I don't see how Us can even compare.
If summoned as a familiar, I would hope they would add the other abilities of the Intellect Devourer that aren't currently in the game (such as the ability to teleport into a hollowed out skull and pilot a person as though they were the brain of that person) which would easily overpower the Imp in utility.
What do you mean by permanently summoned? The Imp pretty much is unless you long rest, or it gets killed in combat. I like being able to resummon it because anytime it takes any damage I can just resummon a fresh one and it's back to full hp.
What would be the benefit to being permanently summoned?
I mean permanent as in you have to keep the damned thing alive or else suffer the consequences of losing your familiar. It isn't a benefit, so much as an added challenge.
