It's fun to catch a noob with his light vehicle not paying attention to incendiary rounds but at higher skill levels, that ability just doesn't pay off vs vehicles. It would be better for Ostheer if they could use the burning bullets later during bigger engagements vs shocks and so on and got better suppression earlier. It has a slightly better than marginal effect vs troops and at max range I don't think it does much.
That reminds me, I feel like HMG42s should have an indicator on the squad they're using incendiary AP ammo in addition to letting the buff stay on them despite packing up. Counting the Shocks and the smoke grenades that could be throw at you and correspondingly holding back on incendiary AP until you've seen them all was pretty feasible against the Soviets...against the Americans, it's kinda impossible with the metric shitton of smoke options they have (Riflemen, Lieutenant, many vehicles including M20, Scott, Sherman...). So I'd think if you manage to micro your HMG42s past all those smokes and keep it on the field firing, you should have the chance to use incendiary AP without waiting for ALL OF THE SMOKE to drop, especially since you've apparently kept it alive through a reload timer in combat.