Flamethrower deals damage per second not like a dot.
You'll deal 56 pps (potency per second) + 5 heat with Spreadshot, 80 pps with Flamethrower.

When you get Scattergun, it will be 60 pps + 10 heat.

I'm not sure about it but I believe Flamethrower can't crit and surely can't direct hit.
It also interrupts your auto attacks, so that's to take in account.

Overrall it's roughly the same value using Scattergun rather than Flamethrower and less a headhache.