There probably aren't enough actual weapons in an mod or video game for more than a handful to be 'underrated' by most people, but there are a few weapons in FH2 that are ignored or catch some shit for being useless online that I think are brilliant.
First for me would probably be the Lewis machine gun. Ugly as hell but I swear it's the most accurate MG in the game to me. I was messing around with it on Tobruk in single player and killed 41 bots with 47 rounds, it's so easy to aim and almost always 1-shot kills in my experience. Wish it looked cooler, but you can't have everything.
For vehicles I think the little Vickers is pretty underrated. I find it used poorly online a lot but it's great for pinning enemies down as much as it's great for zipping along in front of tanks and creating smoke screens for your team, and on maps like Olympus it's even useful to hide in the bushes and then throw a ton of smoke on enemy tanks to blind them. It's small enough you can use it to close down alleys and roads and zoom away quickly if things get too hot. Plus I like firing both MGs at once, as it makes me think of Christmas
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Any weapons you like using that most people don't?