Health down = temporary lower health pool. The rancor does it too. The advantage, other than having an extra debuff for Nihilus’ drain force, comes after you exhaust an enemy character’s protection. Less health = fewer hits to kill.
Thrawn’s Ebb & Flow makes Thrawn a much less viable target when anyone’s fractured. He will rarely get debuffed and always counters. It also keeps the fractured character out of circulation a bit longer if Thrawn has other Empire characters on his team.
Thrawn’s is probably useful in a wider range of situations, but Nihilus’ seems like it can be worthwhile in arena.