As for simcraft.. its been well know throughout mop(5.0-5.1 and again here in 5.2) that a proper firemage will do consistently better dps than the simcraft. Simcraft as far as i know does not use hs+hu either.
I know if your undergeared it can be rng. but thats not the point you should not be undergeared.. If you still feel like fire is Rng, your either undergeared or playing it wrong, cause i am not seeing it. Combustion is such a little part of our dps, that improving on other things such as bomb snapshots and other things such as movement, ignite/pyro spread, when to use hs+hu and when not to etc etc, will make a bigger impact. Another thing that will make a bigger impact on your dps than "rng" is boss mechanics.
But hey this discussion is not new, ive had similar ones during 5.0 where people also claimed rng, and i dont know why cause i am not really seeing it, sure there is small swings but nothing like you claim. But at the same time most people during 5.0 was not taking elpadrino's hs+hu findings important to the point where even EJ disregarded it, and infracted me for false info(lol). So it was not strange that people called Rng when they effectively missed out on ~25% hs's.