Falls victim to many of the same overused tropes that plague Korean shows, and that I have mentioned numerous times in other reviews. Namely...
Minus (1) star: Action scenes which are completely and utterly unrealistic. 30 to 1 odds, yet the bad guys can't hit the broad side of a barn, while the protagonists can't miss a single shot. "Sniper" rifles leaving scratch wounds. "Snipers" missing from 15 or 20 meters away, which is un-scoped pistol range. The Crouching Tiger, Hidden Dragon physics defying karate and leaps from building to building. The list goes on.
Minus (1) star: Unlikable tacked on secondary characters, who we don't care about, their back stories are boring, and we wish would get killed off (or at least punched in the face and told to STFU). But they never are. Wasted time that could have been spent on the dynamics between the two brothers...which is never fully explained.
Minus (1) star: If it takes an entire episode (last one) in flashbacks to explain what just happened in the prior 15 episodes, then you aren't doing your job right writers. The viewer needs to be able to figure some pieces out to make them emotionally invested in the outcome, while still surprised by the ending. But, in this case, you knew all along the genius protagonist had some trick up his sleeve that would save the day. Now, that's fine if it was one small thing. But again, they needed an entire episode to go back and show what he did to make the ending turn out the way it did. Lazy writing.
Minus (1/2) Star: For the whole second ending to be driven by a character simply because he is butt-hurt by his status in life? Jesus, this is so lame.
So, 6.5 stars. All, in all, enjoyable on the whole. Lead actors are excellent and develop some "unique" chemistry between them. Gritty and unique take on "time travel". The actress who plays BingBing (and is also in Money Heist: JSE) is very pleasing to the eyes. But too many unresolved plot threads, and too many convenient encounters to make this rise to the top. Plus, a tacked on "happy" ending that makes no sense, typical of these shows.