Its not really constructive tho. OPs comment is literally the plot of several K-Dramas. Playing into that trope not only devalues the author's wok but it also just becomes a stagnant story with a predictable ending. At least the current plot has substance and is unique in its approach. I've read my fair share of literature as I am part of the academe and as of the moment, aside for actual details and polishing to make the story more linear and cohesive, there's really nothing to critique here unless you're someone who wants to read something that's very similar to the current trends and a million other stories and shows. Adding what OP mentions makes this less marketable in the sense that the market you're selling this to has moved on to dramas and shows. No one reads tropes like these anymore unless they offer something unique, ergo what you're currently reading. :) Tho in my opinion, this comic is a hard read specially when it comes to actual plot. It doesn't offer a wow factor, nor does it offer somethings that's never been drawn/written before. It has its pros and cons but honestly, it doesn't have a spark. Character-wise, the author's seems to just make characters as they progress and that is a sign of lack of complexity and actual plot. Those would be my constructive criticism. ^^
Personally, I haven't even finished the story but their comment is pretty much constructive criticism. They aren't bashing the work or just commenting for the sake of negativity. They're offering genuine ways that the manhwa could be improved. I don't get what you mean by generic k-drama plot because the advice details from that standard entirely. It literally focalizes on already existing fantastical elements that could be further explored. The manhwa itself just centers on the romance between the two characters, and there's nothing wrong with that, it's a genre after all. But that doesn't dismiss OP's comment or render it negative because it doesn't align with what you think is good literature! Also, I wish you could elaborate on what you mean as the other trending works or typical K-drama plot because that's really vague and I'm not really sure how to fully delve into that

Genuinely the author missed the potential of writing a good story, u literally have a kingdom with flying dragons, a tribe of ppl with horns and that was the best scenario u could muster to make a fantasy love story. This easily could have been a village that needed to pay tribute to an emperor after discovering a mystical village. A few of them could have been added to a harem and some could have been servants where our MC is introduced but the emperor notices him and adds him. Then the toxin could play into the affect of forbidden romance if the imai weren't aware of their toxin. So much wasted story