Flamethrower is a DPS increase under certain circumstances. If your option is to Scattergun for the next 10s and your Drill won't cap charges and Air Anchor and Chainsaw aren't coming up, and Wildfire isn't due... if you can safely stand somewhere for those 10s, it is a gain. Scattergun is 640 potency for 10s. Flamethrower is 800. But Scattergun would gain 40 heat. That shrinks the gains but not by all that much.

Given it is a capstone ability, it should do something better than it does.