Both are about people who go back in time and become important people. Both have tragic love stories.
Person from our time travels back to some Chinese historical period. Knows what is going to happen (on a large scale) but never tries to change anything and mostly just watches and participates as if he/she were part of it from the start. Very similarly the first third of both series is really exciting and fun to watch, the second third is serious and tense, and the last third is a mosaic of pretty much just different sad endings, one way or another.
The drama-version of The Myth has a lot in common with Ultra Reinforcement. Both star Chinese Paladin actors (Hu Ge and Wallace Huo, respectively). The premise is similar. A modern guy stumbles into the past and changes things around him by bringing knowledge of the future, similar to Connecticut Yankee in King Arthur's Court by Mark Twain. But while Ultra Reinforcement stays a parody, The Myth builds an epic-long story and keeps the lead in the past for a lot longer. Unlike Ultra Reinforcement, the character in the Myth ends up being changed by the time period. Also, there's that whole switching back and forth between the lead and the people in the future trying to figure out where he went. If you're intrigued, try it out.
Both series are set in the same period in Chinese history (Qin Dynasty) and features the political turmoil of that era, though both are fictional and belong to the wuxia, fantasy & adventure genre. Both have a male lead that started out innocent and carefree until circumstances forced them to mature and take on the responsibilities of the world. The romances are generally tragic but in The Legend of Qin, you'll get a happy ending or two. Good story lines.
At first, it seems like the shows have little in common. The Myth is a time traveling-adventure-comedy drama and Nirvana in Fire is a historical-political-war-revenge drama plot. Although both portrayed by Hu Ge, the respective leads appear very different, Yi Xiao Chuan (The Myth) is a childish troublemaker from modern-day with a penchant for womanizing, while Mei Chang Su/Su Zhe (Nirvana in Fire) is the mature, elegant gentleman strategist. At first, I thought the Myth was only a time-traveling Hu Ge comedy that didn't take itself too seriously. Then, Yi Xiao Chuan's character started to change. As Xiao Chuan's role in the drama changes and he starts to be involved in the court and the battlefield, the drama becomes a lot more like Nirvana in Fire. There's political intrigue, battles, strategy, and inventive plots. I'm at episode 38 right now and just raised my score from a 7.0 to an 8.5. If you liked Hu Ge in Nirvana in Fire, give The Myth a gracious chance. You may be surprised.