That is 100% true, but anti-bunker is only of use if we are attacking fortifications. Larger flamers on armored vehicles would be usable for suppressing infantry in trenches, a much more likely situation. Also a flame tank can mount a high pressure pump for more range. More range == better. A flame tank could also suppress a bunker by targeting it's firing slit...
Consider the M4A3R3 (sounds like a SW droid...), it was awesome against bunkers, and it had far more range than an infantry flamethrower. Mostly the flame tank would be good to lead infantry assaults (clear enemy trenches).
Infantry flamethrowers are too bulky for fast movement, and they are very visible, marking that guy as a priority target. That would limit them to urban or jungle combat (short sight lines). Flame tanks would be very effective until good anti-armor was a thing.
Further though leads me to the conclusion that we should skip flamethrowers and go for thermobaric shells, in world war 2 the germans experimented with thermobaric bombs using coal dust for fuel, so it wouldn't be totally impossible for us.
EDIT: Here have a quote from wikipedia:
Finally, the flamethrower's effective range was short in comparison with that of other battlefield weapons of similar size. To be effective, flamethrower soldiers must approach their target, risking exposure to enemy fire. Vehicular flamethrowers also have this problem; they may have considerably greater range than a man-portable flamethrower, but their range is still short compared with that of other infantry weapons.
Also I found these ranges quoted:
25 yards for an infantry thrower (flammenwerfer 35)
"less than 150" yards for a flame tank