There is one that gives it to Echelons. It doesn't matter if its a Rifle or an RE, a flamethrower decimates units in buildings. In addition, if you also tech nades, your RE's get smokes as well. Otherwise do that with your mortar.
I don't get the point what flamers on riflemen should provide except for making the strongest AI standard inf even stronger. We had that a while ago and it was simply ridiculous.
It seems like you search all you answers in improving rifles to the point were they can handle every other unit. But that is not the point of this game. Even USF needs (a smaller) amount of combined arms. AT guns and MGs HELP but are not necessarily needed. You can wreck AI and AT with your weapons upgrade, but way less consistent.
Look at the flaws of other factions in order to see, that every faction has its problems.
Soviets: Have hard AT problems if going T1
OKW: No indirect fire or smoke besides going Med HQ
Ostheer: No reliable counter against light vehicles if choosing anything but Mobile Defense Doctrine
USF: No snare, no indirect fire, no flamers, unreliable handheld AT, justkillmeplease
Flamers on mainline infantry is dumb period.