I'm hoping it will be an add-on that isn't used in more 'competitive' style games, simply because it's TOO good of a disruptive tool. Yes, technically speaking both players have the potential to tale advantage of it but that will be so lopsided the spell immediately breaks certain match-ups.
EVERYTHING FROM HERE DOWN IS ASSUMING TOURNAMENT STYLE COMPETITVE PLAY
For example, Arkhan Death Vs. Blightking focused Maggotkin, both reasonably strong mid-tier lists, and yet it should be immediately obvious just how 'boned'(heh, pun) the Maggotkin player is just off of his one spell. Arkhan can pick a unit within 18" of him every turn and just blast it off into space, and with Blightkings being both expensive and slow(even with Maggotkin benefits) it takes them almost entirely out of the game. The spell could be rewritten as 'pick your opponents most expensive unit, it is slain' and it wouldn't change much for the maggotkin player.
Yes technically the maggotkin player could unbind, or Arkhan could miss the cast but in reality those are both such ridiculous longshots due to Arkhan having as much as +6 to cast, that not only will it never miss, the Maggotkin player will never be able to cast it themselves. Yes, it's also possible that there are other things for the unit to do outside that 24" bubble, such as contesting objective, but the thing is the unit was where it was for a reason.
That's an extreme example but you have this same problem all over the place, You brought a unit of 15 brutes? I brought a 100pt spellcaster and got arcane terrain. You brought 20 retributors? I brought Morathi and got Arcane Terrain. That one spell is so powerful that it crowds out large units and slow units just by being a POSSIBILITY. You can't afford to run large units or large slowish monsters or even slow units because if you get hit with banishment it's only one step down from just removing those units from the game.