I'm having the same horrible experience with reactive. Was the first interface I've ever crafted and it's total garbage. Tested for an hour running around dark Astoria with either my enforcers or my bruiser.
Proc rate felt all over the place, but was never remotely good.
The most common occurrence was one instance of 13-16 dmg per fight. Maybe one in 10, there'd be two ticks. Only once in an hour did I see 5 ticks. I would guess it's also shared cool down, because it didn't seem any more or less frequent with the one bruiser out vs the two enforcers. Tried resummoning and re equipping Reactive, to no effect.
It's basically the worst ability in the game. About the same damage output as brawl, with a tenth the frequency.