>>713282582It's honestly really not. It's low damage on a direct and can only end up doing significant damage over the course of 7 seconds.
It is genuinely difficult/unlikely to ever kill anyone with flare spam with any of the flare guns. And the healthiest classes can generally just shrug it off. A Heavy isn't going to unrev and run away because he was hit by a stray flare, nor is a soldier going to retreat, etc. It really only messes with light classes.
Pyros only spam flares because it's the only thing the class is capable of doing during longer-than-flamethrower range engagements. If one wanted to sit back and spam explosives all game, they'd go demo or soldier.