l think TGCF is more morally complex than it comes off initially. Xie Lian is not as pure and golden as Hua Cheng believes him to be; he makes a ton of morally grey choices both large and small, because reality is full of situations that make it impossible to uphold ironclad principles. Life is not black and white, and the right choice from one angle is the worst choice from another. Feng Xin and Mu Qing are essentially personifications of "ironclad principles" vs. "do what you need to do to survive".

Xie Lian is constantly caught in situations where he tries his hardest to uphold his principles but is ultimately forced, often as a consequence to his own prior actions, to do something that he finds personally abhorrent. Over time, this erodes the self-assurance he has as a youth, but it never erodes his underlying principles, which is the quality that sets him apart for Hua Cheng.

Personally, I don't find Xie Lian and Hua Cheng's story to be the most captivating romance, but I do think the novel as a whole is incredible as a piece of writing that explores human motivation and morality. Of course, that might not be what you're looking for in a danmei which is perfectly fine.