This happens with many anime. Especially ones with powerful characters, the Fandom starts getting it in their head that the side characters or most other characters are weak. Which for the most part they are not. Literally what we see is the peak and beyond that of ability represented in the main cast and the top villains. Everyone else is what is the average or above average for the world. It's like our world with super genuis kids. The other kids are not dumb, they are the standard or above standard. The super genuis are just a whole other breed.

That's exactly what is here. If you remember correctly Gillthunder and Howzar were taking on an army of giants, alot of the humans in the past took on agents of stigma and killed a good deal of them.

It only comes down to the top clans. Demon and Goddess. That the humans and even other species look like trash. However even then you gotta remember that it only took three people to take down the Red demon in season two. As in Gillthunder, Howzar and Griamore. Then with Hendrickson they took out a white one.

Even if thet aren't as strong of the ten commandments they are no joke.

Then, like I said. You must remember who their enemies are, and who the protagonists are. King is the absolute top of the fairy race. Meliodas doesn't even need explaining. Diane...can't say much but you know she's OD. Merlin was a normal human once and had so much potential as a freaking child that she pulled a fast one on literal deities.

Then we get to Ban and Escanor. Who show that the other races have access to some very dangerous things like Graces or the fountain of youth. Pair these up with anyone and some proper training you have an unstoppable force.

It's true in this show humans aren't all that represented in terms of strength without outside help or something esle backing them. Though I do believe it shows one thing. How fucking good we are when we have the right tools.

Anyway to finish this off, sure humans aren't competing with the top of any of the other races, without a special factor in the mix, but at the end of the day they still can go toe to toe with plenty of the other races. Their just less durable. Except for possibly faired.