I think the flamethrower could've been used alittle more often than it was. I felt it was weak for how rare it was to see it.
you mainly get to see it on flood levels, theres 2 on cortana, 1 on floodgate and 1 on halo, theres only 4, all on flood levels, you're right, they could be fun to use against grunts, they run around screaming