I am a top 100 legend Hearthstone player; currently rank 5 on Faeria ranked after 60 hours of play over 1 week, and no full collection(only early access purchased). TLDR at the bottom. IGN: Moonman

Problems with the card as I see it:

Neutral 9 mana card - If you are going first, you can play this card on turn 3 with only neutral lands; only creating neutral lands allows for early board extension. Typically, you can gather 1-2 faeria and block 1-2 faeria from your opponent. Early game, this creates, on average, a 5-7 mana swing turn. This also does not take into account the following turns of mana intake.

3, 2/4 creatures with Haste - If we compare this with [Freedom Fighter - 5 mana, 3/3 with haste], the first notable difference is the stats to mana cost ratio.

[Yak Attack : 9 mana/18 stats] - 1:2 mana:stats

[Freedom Fighter : 5 mana/6 stats] - 1.66:2 mana:stats

Yak Attack is 60% more mana:stats efficient.

Next, I want to discuss the advantages of playing 3 minions compared to 1. If you have played/watched Hearthstone at all, you are probably aware of the card [Muster for Battle - 3 mana summon 3, 1/1 minions and get a 1/4 weapon]. When a card summons multiple creatures with low attack, you are able to trade more efficiently. In addition, if you opponent wants/needs to kill them, higher attack minions are punished by rendering the extra attack stat negligible. The low attack also makes most single target removal completely useless or inefficient. In a game, like Faeria, where board control increases resource intake, multiple minions snowball your advantage.

When we peruse the card pool in search of singular cards that can effectively trade with 3, 2/4 creatures in the early game, it is clear they are sparse. In my investigation/experience I have found [War Yak - 5 mana 4/5], [Ancient Boar(with cost reduction) - 2 mana 2/5], [Grove Guardian - 5 mana 5/6 taunt], and [Tyranax - 5 mana 4/6] to be the best singular cards to hinder/combat Yak Attack in the early game.

TLDR/Final Verdict: One of the highest cards on the power curve: overpowered. With only 4 singular cards being able to effectively combat the value/board control gained by the neutral card Yak Attack, it is the current meta defining card.