Well, the thing had what was already one of the best typings and then had an ability added onto it that makes it have no weaknesses. I believe that that is what Snaquaza is referring to when he says it is fanboyish. It doesn't matter that there's an excuse for it to have that ability, it still comes across as, well, an excuse for it to have a fanboyish ability. Like if I made a Dark/Poison mon and gave it Levitate and responded to complaints that it seemed artificial with "Ah, but it floats. Integral part of the design, that. STOP MESSING WITH MY ARTISTIC VISION!"
...*Ahem* And Bug/Steel w/ Flash Fire is better than Floatydarksmog, so yeah.
Plus Bug/Steel is so overused. We've had 6 of them in 5 generations. We're ahead of the quota.
Edit: The point I was trying to make is this; Bug/Steel with a Fire immunity looks really artificial. It doesn't look like an actual Pokemon, it looks like someone has gone, "I should make a Bug/Steel with a Fire immunity. That has NO WEAK."