Forms a cycle with Modron March, Blood War, Will of Zerthimon and Planar Expedition.

I just realized that flickering is hard removal against creatures cast with portal.

That's an interesting point about the interaction. Portal in general does leave open some complexities with anything relating to exile. This does seem to create an interaction problem for cards like Planar Journey and Into The Maze. I wouldn't inherently find it worrisome with regard to more general interactions with the rest of MTG. But it does bug me for Portal creatures (of a non-returning variety) and these cards to be in the same set.

I'm not sure if a way around it could be built into the mechanic itself. Definitely gives me something to think about for the set, since it's a fairly fundamental issue. Not sure how to address it short of either completely changing or scrapping Portal or leaving all flickering out of the set.

As far as this card in isolation from that, I have some trepidation about the first ability in general here. I guess there are cards like Astral Slide, but repeatable flicker with no hidden costs is pretty significant. Cycling at least has an implied cost. Gates are lands though and don't.