To cite Clarke's Third Law : "Any sufficiently advanced technology is indistinguishable from magic."
I've always thought one of the brilliant aspects of KK's works is that the magic operates within definite limits. Some of limits are plot driven; the persecutions and the attendant need for secrecy/discretion are one such example. Other limits seem to be more universal and grounded in the material world. Deryni can't just do high-powered spells or rituals repeatedly with incurring fatigue or exhaustion. Spells can have physical range limits (like the Call that can't reach across the whole kingdom or through solid rock). Such limits (and characters dealing with them) are a huge part of the drama.
Your Mileage May Vary, but I find that these "rules" permit me to enjoy the books eventhough I'm a freethinker and a secular humanist. If everything the Deryni do can be explained on a material basis (say as the products of stronger than usual bioelectric fields generated by their brains), I'm fine with that. Consequently, I enter into the game of puzzling out how the magic works vis-a-vis the material world as we know it, and it seems many other people on this site do that too. If new scientific discoveries could be put towards an understanding of how stuff works in KK's world, that enthuses me even more. Sure, to the characters it's magic that Deryni can light a candle by snapping their fingers because they don't know about molecules or heat or friction the way we do. For myself, I want the "deeper" understanding of the "science" behind it.
It's not that I'm incapable of the willing suspension of disbelief; I'm already doing that to go along with the quasi-scientific notion of Deryni brains producing stronger-than-normal bioelectric fields that can have effects in the world outside their bodies. It's just that I like having something more.