[Radek]

I can confirm Bsftcs' findings. I can also create a portal almost everywhere and anytime - even in the middle of a fight. The only place where I cannot create a portal is the Pyramid.
As far as Bungleau's theory is concerned, I don't think it's a matter of timing. I am always checking my portals if they are too far from a town. Therefore, I must create portal, go away from the portal and cast Teleport. This should be enough to "stabilize" the portal. Nevertheless, the Create Portal spell can create nothing.
As far as unfriendly places are concerned, there is one in the game, which is super unfriendly. Sail to the south from Bloodbeard. You will find a small piece of sand immediately before the area with 3 Sand Crawlers and the Altar of Serran. Create a portal there, heh? It is possible to create a portal there so that the unfriendness isn't absolute but you will need 3 or 4 attempts for it. Why? I don't know. Another unfriendly places are the stripes of sand between the "forest" and the sea in Brimloch Roon. Sometimes you succeed, sometimes you fail. Is it related to a "difficult terrain"? I don't know but I don't think so.