Technically, this counters 'uncounterable' spells. Maybe that's fine. I don't know. I only know that Wizards goes to lengths to make sure that doesn't happen (the wording is usually "Counter target spell. If you do, exile...") See also, Dissipate or Syncopate.

I think that's the point. Counterpoint, see Summary Dismissal

"Exile target thing or non-thing".

Part of me wants to say "Well, this being in play is an annoying lockdown threat".

Another part of me is saying "How do I get an infinite number of these into play?"

Also - non-land permanent maybe?

Ah, true. Summary Dismissal does that. Carry on.

­Mindbreak Trap did that almost a decade ago. We also have Unsubstantiate. I have to say that I don't actually agree with the principle that everything is fine as long as WotC did it. You can end up making all the same mistakes they do - and they do mistakes a lot - or end up not doing something that they should have done ages ago.

Esper color combinations prompt this design from a designer quite easily. I made myself a "Exile target spell or permanent" instant years ago. Making it a seal is a nasty twist.