Hellions deal 8+16 vs light with pre-igniters at 2.5 attackspeed.
Hydralisks deal 12 damage with an attackspeed of 0.83 at 5+1 range.
So, vs light, each hellion deals 9.6 DPS against each target, while Hydralisks deal 14.5 DPS. Hellions are 100/0/2, and Hydras are 100/50/2. Even with absolutely no micro, they are still almost even.
Now, you should notice that Hellions deal damage in a line. Meaning that if they close into melee range against a hydra ball, each one should be hitting around 4-5 hydras (bit conservative, but still).
This means that each Hellion should be dealing at least 42 or so damage per second, if not more. This is nearly triple the damage of a unit which costs more resources and has less HP. They also move faster than Hydras, even on creep.
So, yeah. Hellions only suck if you are using them at maximum range/don't have preignitors.