Even when the flamethrower is hitting an enemy, Overload, etc. sometimes fails to set off an explosion. Talking about the geth turret. I still think it's worth speccing into for the possibility of fire explosions, but they won't always trigger.
Turrets aren't really useful purely for their damage. I'll spec out my sentry and geth turret to do the most they can, but in the end as long as they draw some aggro and stagger a few enemies I'll consider them worthwhile. Combat drone in particular is amazing for drawing aggro because you can set it up right next to enemies. It's my favorite anti-phantom tool in the engineer arsenal.