I there any added effect or difficulty in casting a spell across a regio border? I'm thinking an AC spell like scrying or an intangible tunnel. I can't find anything, but it seems reasonable that there might be some difficulty. What does the group mind have to tell me?
Maybe you can add one more to-roll die in case of a botch ? It seems enough to me.
I expect a similar reservation for scrying or intangible tunnel spells: no problems in general, but be aware of particular circumstances.
Some inhabitants of a regio may have powers to detect scrying and/or tunneling from outside, and to resist or subvert them. AFAICS, the most likely are faeries with powers like RoP:F p.61 Extended Glamour related to the regio. But some saints or demons associated with a regio might have similar capabilities.
Also, it's reasonable that the nature of a regio prevents those spells from working across a boundary, or through multiple boundaries. I think there are some examples of such regionnes that have been mentioned in the books, but I might be conflating that with how regionnes work in sagas...
We know that ACs stop working when the they represent crosses into the Magic Realm. It's not inconceivable that this property might radiate out through a regio, in some way, especially the closer it is to the Magic Realm (aura magnitude). But there isn't anything official.
Well, you'll find nothing in the rules books stating that regiones are a kind of different world, nor the contrary. So the simplest and more accurate answer is probably "it could be like that... or not".
Only four "parallel worlds" are described in the books : magical realm, Paradise, Hell and faerie world.
IMS, I decided that only the frontier with those parallel worlds always cuts arcane links.
This opinion is comforted several times in the books.
Faeries p. 16, states that an arcane link to a thing present in a regio allows to find that regio's frontier. Well then, you could use the arcane link...
Against the dark, p. 59, states that nothing forbids to cross a regio border using an arcane link, you can also teleport. But the text adds that some regiones are not reachable with Rego magic and there is no reliable way to tell if Rego magic could reach a regio.
Tales of Power, p. 57, has a "pile" of regiones and arcane links are cut only at the highest point (level 10, magical world).
Thanks everyone for their responses. I have an idea for how I want things to work in my saga,