It's buggy as hell. Don't use it against any enemy that can knock you back or inflict prone - it completely bugs out against those. It also sometimes bugs out against enemies that make multiple attacks. I've also seen it inexplicably fail against the phase spider matriarch for no apparent reason (the animation plays and it's used, it just does no damage).
Anyone else unsure how reactions work? I cast hellish rebuke and it added a reaction option, but won't activate it when i get attacked.
You have to actually be struck for it to work. Misses won't trigger it. Here is the spell text: "The next time you take damage, you use your reaction to surround you attacker in hellish flames that deal 2d10 Fire."