Prior to the heavy nerfs these weapons received they were the embodiment of high-risk high reward with far too much reward in comparison to the risk since it was near guaranteed to net multi-kills in the 20+ ranges once even a single flametrooper managed to get close enough to the objective.
Now it struggles to kill 2-3 guys, sometimes even a single one under the worst of circumstances.
These weapons are meant to deal with entrenched units such as bunkers, houses and trenches, either burning or suffocating most of its occupiers until however few are left can be gun down as they panic out of their defensive position. Unfortunately after the nerfs they lose the near monopoly they should have for rooting out infantry from its hiding spots.
In their current state they are superseded by all other available options.
Ever since the early days of the game they were always an easy target, a human candlestick that anyone could and would target, really the only victims being the hordes of bots since they used to be far worse. This trend continued with the buffs, though now becoming an actual threat which needed to be dealt with by actual players. The tweaks to how fire damage, namely the extra time needed for a soldier to catch fire and the ability to pull the trigger once set on fire, helped those on the receiving end a greater ability to react.
Issue however was often that the fire cone was too wide, too many would get set on fire at once and without the ability to react making the flametrooper hard to hit as it still could properly move to cover or a more advantageous position. Though typically it was still rather doable to kill flametroopers from the front.
Now the nerfs narrowed the cone and made the flametrooper nigh immobile whenever using the flamethrower, which would be somewhat acceptable, though rather clunky and still unfun to play, if the TTK of said flamethrower was at very high. But the flamethrower is neither a single-target weapon that will kill fast enough to reduce the risk of return fire from whatever it is dousing in flames, nor can it really deal with groups of enemies as the slowly improving AI and its inability to maneuver make it an easy target for anyone who is close by - and typically a nuisance for its allies once close-quarters are involved.
As matter of fact, the flametrooper’s major saving grace to maneuver since the moment it stayed still it would be picked out either by a sniper or the infantry that it was dousing in flames, and more so its own flames. Since it cannot really move while firing it is even more prone to set itself on fire once it gets near or inside a building, with the latter case ultimately resulting in a rather limp-wristed kamikaze attack - which was already prevalent before the nerfs, but now it encouraged since it is still the only way the flametrooper can be of use.
Couple this the general underactivity of the flametrooper given that it typically has to change weapons as it gets in range and even then there is still a mild delay before it can start flaming as well as any residual flames on the environment being rather inconsequential since most infantry can run through them unless the environment is especially restrictive - and even then the tick damage is fairly inconsequential since there is still a roughly 4s window to put out the fire which get increased with vitality, which only worsens the flamethrower’s TTK. Without mentioning its already deeply restrictive ammo pool.
Want to deal with groups of infantry? Machine guns, SMGs, Assault rifles, AT guns, Tanks, Planes etc. are all better options.
Want to storm a entrenched point? Artillery(if open), guys with SMGs/ARs, Tanks, Planes(if open), even White Phosphorus, Molotovs and grenades are all superior options to using a flamethrower.
Even whatever defensive capabilities it may from the flames being able to lock down avenues of attack is pretty neutered since they can mostly just act as sentries whenever they anywhere close to their engagement distance.
Also the older flamejets looked way better than the current ones.