Actually for some situations, the flame mech might be heavily underpowered. The lowered basic attack range is *bad* against anything that fires enough crap at you. For some bosses on higher difficulty, it's likely to make the fight 5x as difficult because, well... good luck getting close enough.
Unless they changed the range on the flamethrower, that is, since I had a go with it last. But last I used it, it wasnt good. My rule of thumb so far is: If the weapon's range is shorter than that of the White Gloss Mech's basic gun, it's going to probably get you killed in high-difficulty situations. Normal mode though doesnt have too much of that. Probably.
Now that being said, weapons that do alot of rapid hits are very OP right now as damage goes. This might be one for Mantis, because it's a problem that seriously needs emphasis on how bad it can get. Late-game boss HP is high right now, and this is the reason.
And it's possible to get that kind of damage with very LONG range weapons. Early in the game.