I'm going to be the one making the rating go down lol, because I've been scrolling for a while and all I can see are 10-star reviews.

To start with what I liked:
- Acting? Good. It immerses you in the story, it doesn't feel forced, nor scripted, and the chemistry is there (for all the characters). I love Lili especially, she really gave off younger sister vibes.
- Cinematography is also clean and curated, good color grading, and generally visually appealing work. I liked that.
- I guess the main couple has a lot of cute moments, you can really see the affection and love they have for each other, the way they would die for each other if needed. Their bond was apparent and strong.

The elephant in the room:
This is just plain incest. Let me compare it to Stay With Me, which also had some incest-y tones to it: there, the two guys meet as teenagers, not knowing they are related. They become friends, maybe something more, then find out they're stepbrothers. So technically they're related, but haven't grown up together, and don't see each other as brothers. Even though that's still a questionable premise, it isn't as scandalous as this one in Unknown. Yuan has been *raised* by Qian since he was a child, and sees him as a hero (and older brother). Qian acts like a brother and father figure to both Lili and Yuan as they're growing up. I could see Yuan maybe developing feelings for Qian as some sort of unhealthy coping mechanism, but in NO way would that make it acceptable for Qian to reciprocate those feelings. He is YEARS older than Yuan, has treated him like a sibling since childhood, has protected him from abuse and poverty. He should know better. He is an adult, his prefrontal cortex is fully developed, and he knows (should know) right from wrong.
The fact that they're not biologically related doesn't mean anything. That would make it okay for adopted siblings to engage in intimate relationships. They were raised as such, and therefore it's incest. Point blank. Yuan needs therapy and so does Qian - and maybe Yuan should have stayed in the US. Maybe some distance from Qian would have made him come to his senses. If they had been ANY other thing - neighbors, childhood friends, family friends, coworkers- I would have been okay with it. Hell, as I've said, even stepbrothers can get a pass if the series is good (and they didn't grow up together). But this is just not it. I'm surprised by the lack of shock they received from the people around them. What makes it worse is having seen Qian with Yuan and Lili as literal children, which makes me kind of queasy because they change their actors to younger ones but not Qian's. So I couldn't stop picturing that poor teenage boy falling in love with his adult sibling.
I say this as someone who has adopted cousins. Not biologically related is still related. That's all there is to it. There might be chemistry, there might be love, but it's still weird as fuck.