Mediums can arrive 1-2 minute after a Hezter.
And you can totally lose a game in that 1-2 minutes, especially if you're already behind. Buffing it'd be a great way for a better player to steamroll a worse player even more than they already would anyway, without actually making it that useful in an even fight. That said, I think it's kind of meh.
As someone who extensively used Feurestorm on the lv17 1v1 level and even used it in Kimbo's tournament, my opinion of the doctrine is the following:
Flamethrower Spios: Meme levels of useless, unless vs USF without field defenses ability to lay mines.
MP40 Volks: Good but you can only get 1 or 2max unless you wanna lose the game to MGs. Really strong if you get close or on retreat path and very muni intensive.
Opel: Amazing
Incindiary rounds: You can't afford the munitions. Great vs sim city which noone uses.
Flame Hetzer: Very niche. For a bit more fuel you get the P4 workhorse that can deal with anything short of heavies.
Rocket strike: Great vs concentrated support weapons, but not very wide which means it's niche. Also expensive af.
Overall mediocre in 1v1s. You can get tonnes more value picking another early elite unit doctrine. Not worth picking over Kpen offensive in live, or Breakthrough/Luftwaffe in the preview.
I've never found the incendiary rounds or sturm flamers the least bit frightening at all.
The incendiary rounds just don't do much damage to anything but I guess they at least cover a decent area. Maybe buff the firerate on the barrage to make it saturate an area quicker? I've found that a lot of times the shells will just scatter and miss and I'll just walk around them or pull back without ever even taking damage.
Sturm flamers seem like a weird design decision too since volks already have a flame-related green cover counter that you don't have to give up a minesweeper and repair speed bonus for.
The really nice thing about the rockets is that they are pretty much guaranteed to destroy team weapons fully. Definitely useful if you aren't in a position to destroy/salvage them yourself.