I probably can't rate it since I never got one here. From what I know, it's combo-locked, high ATP, its special sends out 5 strong waves in front when HP is low. Personally that's not my style at all, and it has its potential to hurt certain bosses, but the risk of getting hurt just concerns me a lot.
If I did have to rate it now, my Dark Flow rating is 5/10 mainly because combo locked, and the special's style conflicts with my style of preferring safety. However, I'm sure others will have use for it. I'm personally more content with a Flowen's or Red Sword if anything.