From what I've seen and played, Johnny is the most replaceable member of the Fantastic Four. I don't think he's bad, just that both of his powers are kept in check by tough activation requirements/paying a big price, while the other members of the FF are more consistent.
Flame On 4 - 2 wounds on a 4+ is just insanely nasty, but having to take LEA helps to temper it. Johnny doesn't have any way to get rid of those wounds either, and without Invisible Woman it's easy to get through his 4 defense.
Supernova - 7 Attack to all figures within 2 spaces? Bananas. However, Human Torch gets kneecapped so badly after using this power. I think in a perfect world, you get a few Flame On's, before blowing up everyone around Torch when he's nearly dead. Easier said than done though, especially since he'll usually be in the middle of quite a few enemies after using this. You have to accept that Torch is pretty much dead weight after this power, and sometimes you roll 2 skulls on Supernova against that figure you really needed dead.
Lava Resistant - Pretty situational and map dependent.
Synergies/How to fight - Obviously the Fantastic Four, particularly Thing. Torch will usually be getting wounds on himself pretty quickly, so Thing can select him for Clobberin' Time. Beyond the FF, Crystal's Tidal Blast helps position enemies for Torch, same with Medusa's Hair Assault. Outside of those two Adventurer's, I'm sure Torch appreciates someone that can manipulate his enemies movement, or his own for Supernova positioning. Johnny gets absolutely neutered by figures with Lava Resistant, so if he's going crazy on you, try someone with that power out.