Lord Black, I think you already understand why your idea isn't quite suitable.
codenameray, well done. The story is good, and you've thought through the units well. Let's get started with the review, if you don't mind. The first thing I want to ask is, do you remember Forge? I have a strong sense of déjà vu. And, as with Forge, I'm against something like this in Heroes. We have enough Dragon-Golems from the 4th installment. Second question, were you inspired by Warhammer? The déjà vu is just as strong. Okay, let's move on to the units:
1. Miner-sappers are ordinary; there's nothing to criticize, but also nothing to praise. There are plenty of them in games: like these.
2.

Equipped with two steam fists or pneumatic fists. In short, iron gloves that somehow punch on their own. Unfortunately, I don't know how to explain it to you in more detail.

Does it need to be explained? Here's a clear illustration: this. I can find more examples from other games later.
3. Thrower-bombardiers. Again, it's all logical, but also banal; many games have something similar. Let it live.
4. Icarus. More information is needed; again, the associations are banal: like this.
5. Mechanical warriors and mechanical knights. One, two, three, four. I can continue.
6. Cannon-gunners. Ogres! like these.
7. Mechanical giant-destroyer. One hand is a mace, and the other is a flamethrower? It's hard to find an exact copy, and I don't have time right now; I'm at work. Here, take a look: this.

The hero himself evokes associations with this.

In conclusion. It didn't turn out to be completely new, from scratch. You came up with a great race, but the unique feature is banal, overused, and steampunk. You did a great job in a day, especially considering that you were also working; I honestly praise you, you deserve it. But it doesn't fit into the concept of Heroes – one, it's full of clichés – two. In my opinion, it's a plus/minus, a draw.