1.) Against a mammoth or a medium tank you stand only a chance if you approach him

a) unnoticed untill you get in range of fire and

b) You have full health and armor. If someone scratches your paint before you reach your target you loose.

Conclusion:
A flame tank will always loose if seen early and if the other tankdriver isnn´t a complete newbie. You loose even against a Mrls if spotted early because of their firepower.

2.) Without the flametank Nod has no good anti-mammoth tank. If there is a mammoth camp and gdi has good snipers who take care of the laser chain gun soldiers and MA´s the flame tank is the only hope to get out of this situation alive because some hotwires repair the mammoths faster than the light tanks damaging it.