So, I've had a chance to play the main leaders that people suggest are OP: Cleaver, Eist, and Eldain. Also played a bit of Brouver. I have to say, I've had very challenging matchups against both old and new decks, and while these new cards are very strong, they aren't really OP in my opinion. I'll explain.
Eist requires bloodthirst 2, which is a real challenge versus many decks. Dwarves have tons of armor, SY plays mostly engines, and traps ST barely plays any units. Getting the bloodthirst can be difficult to impossible in a short round. You can also get put in a position where using Eist and leader in the same turn constitutes a massive overcommitment, but not using both results in a large amount of missed points. Or, you play him in round 3 with greatsword in hand, only to draw Jutta. Now you have to discard one, and move something awkward from your deck to your graveyard. Ouch. Also, it's hard to play Eist with last say since you don't know what you're going to draw. It could be a lousy 4p unit, or even a raid.
Eldain is highly vulnerable to a bleed, and can be draw dependent. If you draw all your traps in R1, you can be forced to play them when you'd rather play elves instead, hurting your long round potential. If you don't get round control, your opponent bleeds you of 3-4 traps and you're left with a weak 12 point Eldain in R3. Also, if your opponent is smart, they can tell which traps you're playing and play around them.
Cleaver probably has the least downside, though he does get taken out by Morkvarg and incincerating trap, and also triggers serpent trap. Also a lock/heatwave magnet.