1) The whole secret about being a human and the mask wearing gimmick. Not aging well. After six chapters, there's already like two people who know the secret and two potential people that may know. All female. Which is fine, given the harem tag. However one of the females looks exactly like a human yet is part of the "demon/monsters" and doesn't have to wear a mask. Why does he?
2) The whole him not killing his enemies thing despite being in a war. What's worse, you discover it's not just him, but then in chapter 6 you learn the Demon Queen (I have no idea why they call a female Demon/Monster leader Demon King still when there's a title for a female equivalent of King, it's Queen), I digress. You find out the Demon Queen has also sent out a mass order for the demons to not kill humans or perhaps it's lost in the translation and she simply meant spare as many as she could. Which is a contradiction to what you learn before about the Demon Queen getting suspicious about the MC sparing humans.
Yeah, these two things won't age well and I suspect, at least, #1 will fall away at some point while #2 continues for reasons.