I know a lot of people don't like Daniel because he's suspicious, or dismisses her kids, or gets jealous easily. I think it's because he's the first (apparent) main love interest who's hasn't got flashy colors. Most male leads in both manga and manhwa seem to steer clear of brown hair and brown eyes. Something "too boring" for a fantasy romance. Typically they have blonde, black, or silver/white hair and brighter colored eyes of something like blue, gold, red, sometimes black, or the rare green eyes.
When I read the comments I feel like commenters have genuine concerns with his conduct, but it's foundation is in some part of their subconscious that is convinced he isn't the main love interest. Some of the things they say they don't like about him are things like I mentioned before; that he's suspicious. But in other works like this the male lead almost always take an unnatural interest in the MC; it's how they work. All male leads are suspicious with these things, but eventually the story goes on and nobody cares anymore.
Another complaint is that he doesn't consider her kids, or seems to forget about them, etc, etc. I think this is what proves that you can't please everyone. What I saw was a male lead who didn't define the MC by her kids or status as a twice widow. I can imagine that if he was fully considerate of her kids in all his decisions for her then someone would complain that it was a Mr. Feminist who's so pro-women that here really shouldn't exist in reality. I've people complain about that sort of thing before, as dumb as that argument is.
Also as for the jealousy thing; a lot of authors nowaday still abide by the trope of having times (maybe lots of times) that the love interest gets jealous even if it's illogical or frequent enough to be annoying. Especially in Japan, Korea and other asian countries that produce anime content. As for Daniel? It was the first time, and maybe they barely know each other, but that's the inexplicable allure of the protagonist. Shoujo logic.
I actually really like Daniel cause he feels like a breath of fresh air. Don't get me wrong, I love my exotically colored bishies, but they are pure fiction. Daniel feels more realistic and I lovehis dynamic with the MC despite how briefly they've known each other.
Wow that was the most words I've ever written on Mangadex.