Some people think grenades are better.
1) The flamethrower projects flame, which burns over there, not back at you, and keeps burning if it hits anything or anyone flammable.
While a grenade might miss a firing slit (and then bounce back towards you), missing with a few ounces of gasoline isn't a problem, because you can move the stream around until you hit the sweet spot, and the screaming inside the bunker confirms proper weapons placement. Let's see some Smartypants do that with a grenade.
2) The gasoline is almost always at 14.7psi, except when you're about to fire, when you pressure up. The nitrogen propellant tank is the pressurized one. That's the one that does the work.
If somebody throws a WP grenade on your ass, it's a problem, but at that point, whether or not you were carrying four gallons of gasoline is largely moot. The only real drawbacks are signature, weight, and limited capacity before it's empty. The plusses are that anything within range you can see is going to die a horrible flaming death.
3)A grenade goes "whump" once, and showers a few bits of shrapnel. A flamethrower shoots flaming death into bunker slits from 40 yards away, and keeps burning the ass of whomever you splooged, long after the accelerant dies out. And they helpfully scream like little girls the entire time, and flail about, giving you additional morale kills on your intended enemies, and occasionally the 'splodey things on their body like ammo and grenades provide additional secondary explosions and casualties as they cook off! Yay, ingenuity!
4) It was a de facto antitank weapon in urban combat, because you could take out multiple AFVs with the backpack equivalent of a Molotov Cocktail Launcher. I would love to see what one would do to an M1 Abrams or M2/M3 Bradley, let alone an MRAP, just for research purposes. I think it would open some eyes and shut a few mouths.
5) It's exactly the weapon we should use for firing squads for terrorists captured in wars like Iraq and A-stan.
Note the video @ 1:19
Nota bene that cultural mores are such that despite issuing hundreds to thousands of them, no one from WWII or Korea ever says "I was a flamethrower operator." NO ONE. As Col. Kurtz observed, we teach young men to drop fire on their enemies, but we won't let them write "Fuck" on their airplanes. No one wants their relatives and friends to know that their job was to fry enemy soldiers and watch them dance around on fire. It upsets people at their breakfast.
The jackassical M202A1 flame rocket launcher was an abortion of a weapon to replace the M2, another legacy of the NacNamara DoD Idiot Trust of Really Dumb Ideas. The rockets leaked, thus insuring that if you ever fired it, 70/30 you would go up in the same blast, every time, if the pyrophyric agent didn't spontaneously combust the operator and anyone nearby before use upon contact with air. The best use of that system would have been to deliver them, intact, to the enemy, in hopes they would be deployed.
It's criminal that a bunch of non-combat jackasses and multi-star pussies have relegated hella-effective weapons like Claymores, Bouncing Betties, Toe Poppers, and M2 flamethrowers to the scrap heap, but that combat troops have to wear reflective belts to go jogging. I'm surprised they haven't added a module to the M-4 that lets anyone forward of the muzzle know that a loud device is about to be employed, and to please cover their ears, just before firing ignition, and make BUD/S trainees wear arm floaties before pool training. If anyone is still active, do they still do (sanitized for your protection) jody calls when running, or do TPTB just hold a formation group hug afterwards, and award participation trophies? Buncha fucking pussies...