I find the "tab or not" discussion here pretty silly. The easiest way to say whether smth is tab or not is to look whether you can use it w/o a target. And Slumber is literally blacked out w/o a target. So it's a tab skill.
As for it's balancing and whatnot, as long as it gives high resistance to any further CCs to the target - I'm absolutely fine with it. In other words, if you Slumbered someone - the only effect of it is they're immobilize. You can't damage them, you can't really CC them further. DoTs break Slumber too so you can't just put a DoT on a target and then slumber them. In other words, I think it's quite balanced, especially considering the cast time.
As for pvp, L2's strat was always to just hit your "Slumbered" mate with your weakest attack. Obviously 1v1 that would be impossible (though quite often mobs would do it), but we all know that Ashes is a party game, so it's not really an issue. Imo it's just a party coordination skillcheck. As well as an enemy skill check, because literally any aoe will immediately de-Slumber the target, so the cast would've been wasted.
I see no issue with this skill in its current form. Mainly because it's literally this
Like, 1 to 1 this. The more we learn about the game the more I think that half of Steven's design is just "take this from L2 and put it in the game"