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Your original intuition regarding flamethrowers may be about right.

There are one or two WWII studies that concluded they were poor against AFVs, as it was difficult to get any fuel inside the vehicle. An open drivers hatch gave you a chance, but an open turret hatch was not much use. The best chance to disable a tank was to get fuel onto the rear engine deck - if it had open louvres - because fuel may burn the fan belts and wiring.

Flamethrowers were only slightly better against pill boxes.
When storming a pill box, the Soviets would try to assign as many flamethrowers as embrasures.

Not very good against fox holes, as long as the occupants ducked.

They were most effective against houses and even fortified houses, because they would set the building on fire; and against defenses built into hedges and undergrowth for the same reason.

The main effectiveness of flamethrowers was against enemy morale. They caused only few causalities, but caused quite a lot of troops to surrender.

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