Reavers are great - they are objectively better than Hellions in just about every way. They are faster, tougher (i.e., better save); get a heat lance; have a useful ability; have a higher MSU OC score; and are cheaper.
Problem 2 with Hellions -- they are NOT infantry -- they are "mounted" and have to go around terrain features.
Problem 3 with Hellions -- Also, they do not have deep strike.
Problem 4 with Hellions -- they are OC 1.
Problem 5 will hellions -- the ability is good on paper but not in practice with this unit because to actually use the ability they would have survive a fight phase (if charged at) or survive two fight phases (if they charged). Moreover, even if you get to use it, their 5+/6++ is too vulnerable to overwatch.
Problem 6 with Hellions -- the Baron is still missing.
While Hellions CC is good, it is not great. A 5 squad man squad of incubi is cheaper and arguably better than Hellions (I'll take ap -2 over sustained hits 1 and ap -1). Finally, and laughably, because incubi are infantry and able to move through ruins, there will be circumstances in which the incubi are able to move up the board faster.
[Edited -- Sorry. I'm annoyed that my Hellions do not get to do anything.]