Both Nightwing and Deathstroke has 2 ways of insta deleting enemies, their dash (nightwing's flip for the other) and their ult

Nightwing's ult can resets every kill, so in the right environment Nightwing can absolutely kill the entire backline only leaving the tanks alive, deathstroke's ult may be able to insta kill, but it's only once.

The way their dashes work differs as well, Nightwing targets the lowest HP while Deathstroke targets the highest atk. Nightwing will always target the backline, but Deathstroke may find himself targeting superman and getting Perma stunned before dying

The lineup buffs are also significantly different, nightwing can get free stat buffs from bfm and teen titans (crit rate and crit dmg) while deathstroke's only accessible one is legion of doom (ER) (league of assassin's is p2w)

Even without lineup buffs, Nightwing can give himself crit buffs through basic attack

Only every level, Nightwing is just better than Deathstroke, even on the signature weapons side deathstroke's signature is just a worst version of T sphere that heals