You hyun is stellar single target killer but tricky to use. Her concentrated fire is rather plain but it boosted to be okay by goddess of war passive. The meat of this unit though is the assassination sp atk. It capable to deal monsterous damage to single unit enough to OHKO most unit that not boss or shielded unit. Very usefull to kill enemy before they able to activate their % HP triggered passive
Keep in mind though, As expected as early unit, You hyun indeed have quite a number of drawback.
1st her assassination target a unit with lowest hp. This lead her to for most of time overkill the wrong target.
2nd She is not very durable and evade mode activated at very low 30% hp which not very reliable.
Despite her drawback she still a good unit if you manage to ramp up her survivability via soyuz and healer pairing. She is quite flexible in positioning. If you want her to spam her assassination place her on front line. As long as she not facing 3 row enemy, she can survive long enough untill healer help her. Otherwise keep her in middle coloumn to keep her alive longer.
You hyun is not meant to take punishment. Just concentrate your firepower to her without triggering healer. She also can be tricked to assassinate ignore atk shielded unit like petra and elin.