-REVENGE REVENGE REVENGE
-Prison
-Morally grey FL/AntiHeroine
-Separated mother and daughter
-Trashy men
-Violence
-Noir style thrillers
-Great acting
-Dark and disturbing

- Both FLs are witty and mischievous but also strong and caring to their friends.
- Both FLs share the same destiny to destroy the world
- Both MLs are smart and scheming and they were both prodigys from a young age
- Both dramas have really good chemistry but the romance in the legend is more subtle
- Both dramas have a good balance between fantasy, politics and romance.
- Both were high budget in their time but Kdramas generally stage better battle scenes
- Both dramas have vast fantasy elements and adventure
- Both dramas have interesting characters and a theme of friendship
In general I think The Legend is the most similar to Legend of Fuyao

Nirvana in Fire also incorporates elements of espionage, martial arts and Intel profiteering. Both MCs bear similar characteristics of defining tragedies in their past and play crucial roles as strategists to reform the government, but SFD's Sam Bong is more 3D and realistic.
In terms of acting, suspense, political intrigue, character development, plot twists and character range, Nirvana in Fire falls short in my opinion. SFD is more willing to embrace and ride unpredictability in plot execution which is something that's very rare, SFD is also more lively in terms camaderie and comic relief.
Final verdict from a personal POV, Nirvana in Fire is a snorefest with bland characters and a rating of 9.1, six flying dragons is an all round masterpiece with a rating of 8.8, the contrary opinions are overwhelming so give it a shot.

Both have adventure and a clear picture of where they are going and where they are at each point in time. The chemistry is also great in both, the MC's deeply love and understand each other. There is a strong political theme and several battle scenes but more in singing all along. Both have strong and smart FL's that are independent minded. And finally both have serious and tragic theme but there is more humor in legend of fuyao.