I remember during the Ashes of Outland reveal season how everyone was saying that Maiev Shadowsong was gonna be the new Zilliax and that the card was insane and every deck would run it. And well, it turns out that not even highlander decks wanted to run it and she'll probably rotate without ever being actually relevant at all.
I think this time around the Maiev prize has to be for Scabbs Cutterbutter. The card was insanely overhyped, everyone was talking about how rogue would be tier 0 and absolutely unbeatable and guess what? Rogue ended being very good but nowhere near as crazy as people said and Scabbs is seeing close to no play. You can't even find a deck with at least 50% winrate on HSReplay that runs him, Kazakus, Golem Shaper is just a superior card and more interesting for what Rogue wants to do. I'm not saying the card is bad, it isn't, it's infact a very solid card, but it's just funny to me how everyone was going crazy about this guy and he literally is not played in any decent deck across like 2-3 archetypes.
It's still way too early to make claims like this, and I don't think Kazakus will outperform Scabbs in all rogue archetypes once the meta is settled and refined. But if by underwhelming you mean overrated, then I'll go with Outrider's Axe, because I overrated this myself as one of the top 10 cards in the entire game. But It's just a really good card, not OP.
Outside arena, I haven't seen anyone use Outrider's Axe, Death's Head Cultist, Kazakus Golem Shaper, Southsea Scoundrel. Those are all ones I thought would see play. Everything else is seeing play or not pretty much as I expected. Maybe Primordial Protector, since it was a lot like Spiteful Summoner, but I guess the extra cost has scared people away from it.
It's hard to tell. Once balancing starts happening some cards will get the spotlight they deserve. The biggest loser for me is Death's Head Cultist. I really thought this card would do well. I main a Mage and saw it being implemented as stopper for weapon aggro and a bonus heal. Never happened, it's way to slow to play as by that point where you could drop it you have bigger fish to fry - you're half-way dead with a swarm of minions, a bunch of secrets and a weapon casting said secrets or an opponent with a hand full of 3+ level spells.
Scabs is overrated because it’s a 4 cost card that also needs to combo and Rogue can accomplish things for much cheaper has plenty of draw and likes pen flinging with cheap spells. No one cares about making cards cost 3 less when your whole deck costs 3 or less lol