Such issues are best handled through Compels. Otherwise it's not possible to put a fair cost on Sponsored Magic.
Grevane will never suffer from the limitations on Kemmlerian Necromancy. So if they're intended to restrict the Power, then that Power is undercosted for him.
Meanwhile, Harry would suffer from the limitations on Kemmlerian Necromancy constantly. So if they're intended to restrict the Power, then that Power is overcosted for him.
(This is basically the standard "you can't predict how often this will matter" balance issue everything has, but it's an extremely bad case of that issue.)
Fortunately, you can make that problem disappear entirely by treating the limitations on Sponsored Magics as Compels.
And it doesn't even make Sponsored Magic broken. Channeling and Ritual are pretty bad anyway compared to Evocation and Thaumaturgy. It's the bulk discount at work.
