against USF. firing out of cover, absolutely decimating any and all units with flamethrowers, while taking very little damage from units firing from cover.
I can't speak for Bersaglieri, they seemed alright combat wise to me in my very limited use of them. Lack of a snare (or even threat of a snare) put me off them a lot. But I'm a big believer in the scaling power of Panzergrens.
Guastatori well, they do indeed give pioneers a good name. However in 20+ games of them I've not had one post-nerf where they've felt invincible, esp since Allied mainlines will have a bit of a vet advantage due to 2CP timing. Getting models picked off is to be expected, making you pick your fights carefully. The usual Britblob contains AT boys who make your armour feel nonexistent. Obviously if an Allied player 1v1 Guastatori then they'll get stomped and they should since Guastatori have 440MP, 40MP reinforce cost.
Not sure why you seem to think cover works against flamethrowers? If anything you take more damage because your men are all bunched up together tightly and flamers ignore cover. Historically they even do bonus damage to garrisons.
I do see a lot of ppl dive vehicles onto vet I of them, but that's your own fault for what comes next (AT satchel).