I've only ever used sponsored debt and thaumaturgy. (Separately, not combined)
It worked fine because there was plenty of justification. Ironically, the sponsored debt was adjudicated by Sanctaphrax.
I had an appropriate aspect set up in advance which I tagged and I spent a FP on an character aspect related to my demonic sponsor. I took a speed power for one scene.
He said he didn't really like to do that kind of thing but would 'let it go' this time. Maybe he's getting soft....
But I guess that doesn't really a newer your question. I've done it on occasion with thaumaturgy. Once where the group did a ritual which gave us all pack instincts. It lasted for an entire story arch. (Which took place over several hours).
Modular abilities seems the way to go if you are going to do it often, though.