It can easily be adjusted and its beta. That having been said it is something they need to adress because YES it is an issue if choice of hero can decide as much as it does right now. In TFT and Underlords position matters more and there are more synergies and wiggle room, and you have economy you can use and items. Battlegrounds has far fewer decisions to make and the heroes matter alot more to your wins than underlords do in underlords.
Alright, let me rephrase. It's an issue, but definitely not a
massive issue, as you put it. For several reasons. First being that you can discover a hero. Most people, I'm thinking, will eventually get the 30 packs an expansion to be able to roll between 3 heroes. The odds of getting at least a mid-tier pick are high (even with 2 picks you can go for the better option). Second, even those lower tier heroes are able to highroll and pick up high synnergy minions or have powers to at least carry through the early game, making them semi-useful. Third, and this is where Blizzard also comes in, hero balance is not the only thing that matters. Minion pool matters just as much, probably even more.
Curator is one of the strongest heroes you can pick, because his Amalgam synnergizes with everything. But what if the AoE buffs or regular Amalgam were to be rotated out? Suddenly you can't make a Zoo build so easily, making him and his power much weaker. Jaraxxus is seen as fairly weak, which is mostly because demons are mediocre as they don't have a solid endgame. What if they printed a Mama Demon who buffs all demon tokens? Or they reduce the rarity of something like Voidlord or introduce a demon specific buff of sorts. There's tons of ways they can increase the power of heroes simply by introducing new stuff to the pool.
I agree some heroes need work (Rag and Patches are samey, Putricide does alsmost nothing), but I don't agree it's a massive issue. Other than that, I can't comment on the comparison to TFT and Underlords as I haven't played those.
For fun: out of all heroes in the game, I'd say out of the 24 heroes, at least 17 to 18 are in a good spot, which is a pretty solid number.