The biggest problem with the set bonus is that it's RNG piled on top of RNG, piled on top of another heaping dose of RNG.
Not only do you have the RNG of whether Conflag will crit, you then have the RNG of whether that Crit will proc the bonus. Only then do you then have the additional RNG of whether your Incinerates/Immolate ticks will crit when they wouldn't have otherwise.
It'd be entirely possible to have a completely wasted proc just because Immolate or Incinerate didn't crit as a result of the bonus. 10% crit is not a lot.