It always intrigued me, but it disappointed me.
Lack of reach, lack of damage and poise/stagger.
Situationally great against crystalians, but its unparriable gimmick is only relevant against enemy players.
Overall- save yourself the dex if building Str and go for great stars - it shores up every weakness of the nightrider and then some.
A simple fix would be to give nighrider flail AoE slams that chain back to back, but it would be a nightmare to balance in the pvp meta, so I guess there is no such thing as simple.