Yeah, there's going to be some of the very real concerns in line with those shown off in Turn Coat re: Dresden, when the Wardens were afraid of him. But the Senior Council proper is going to be much more interested in why a being of this sort of power is running around. How Viserys got that strength is interesting, but the why is crucial - or would be, if he was a creature of the DF-verse. Of course, they don't know that and he can't really tell them, so the assumption will be 'godawfully powerful being, presumably human at some point, but who knows, that has been hanging in the Nevernever for god only knows how long'. And now it's woken up, requested a meeting, brought the Fomor in North America to the negotiating table in a fortnight to make that meeting happen, and will be standing there telling them to do a better job.
Consider what that could look like.
My guess is that they would see him as a less insane, idealistic Kemler, a wizard who chaffed at the Laws so much he abandoned his humanity to transcend them and is now working to change those aspects of Council policy he most despised. Perhaps he even had a loved one who was executed for Law-breaking.