My understanding is that, unlike most effects in FFXIV, Flamethrower ticks once per second, and also has an initial tick like ground effects do (which is probably related to whatever nightmare code they cobbled together to create it in Stormblood).

So I think it's doing about 880 potency over 10 seconds, which is still worse PPS than your other AOE tools, but better than your unlimited-spam AOE shot (Spread or Scatter).

You can basically go down the list of options, and if you've used up Chainsaw, Bio, and Hypercharge, you can sit through a Flamethrower for a mild damage gain.

Also, it's fragile as a butterfly, but don't feel too stressed if an AOE makes you cancel it. The gain/loss is mostly trivial when you take into account Heat generation, although Flamethrower does look progressively better in situations where more Spread/Scatters won't be able to get you another Hypercharge before combat ends.