In my mind, places like this would make it more difficult for a wizard to cast magic. There would definitely be a negative modifier to the effective Conviction of a wizard's spell, as the energies would effectively be grounded. Sort of like a Quickening on Holy Ground.
That's not a real problem though. I could see such a place interfering with all supernatural activity. It would probably utterly prohibit travel to or from the Nevernever in that area, make various activated powers difficult to use, and prevent all but nigh-godly (or even beyond godly) tracking and scrying attempts. Not even Cowl could find you there, even if he had a whole hand of yours.
Yes, it'd be a disadvantage to any supernatural living there, but I think since they'd have the opportunity to use those advantages which they were otherwise previously denied by their status, the denial of varying degrees of certain powers to enemies, as well as being essentially untraceable, would more than make up for it.
It wouldn't even be a stretch to say that such places aren't necessarily so much rare, as merely unknown...even to old and knowledgeable beings. After all, initially they'd just seem like areas without magical potential, which means they'd never be investigated, implying that no one would realize they're not just powerless, but ANTI-power. Certainly none of the older members of the White Council would waste their time with such a place, and no other supernatural creature would either...the place holds no apparent benefit. The only way supernaturals would ever realize the nature of such a place would be if a mortal is there and they're after him/her for some reason. But then again, they'd have to be tracking by mundane means to begin with, implying a previously immediate chase that's about to put the supernatural pursuer severely on the defensive.
There's enormous potential for such an idea in concert with the various standard tropes of the Dresdenverse.
There could possibly be a potentially serious drawback to such places though in that mordite could form or appear there. I'd consider that to be a negligible risk though considering the rarity.