I've personally found that, when shooting "practical" (short range, center mass, speed-critical), a vert foregrip does feel more natural from a controllability standpoint.
I'm not ex-military or LEO, and the extent of my "training" consists of things I've learned from a family friend who knows a thing or two and decided to share. I have an AR with a vert grip (mainly just to have one!), but when he was teaching me the Mozambique drills we used his AK. I started with my left hand on the handguard, but he stopped me and told me to grip the front of the mag.
I can say, to me anyway, it just feels more natural to not have your wrist twisted when you're doing that kind of stuff.
When you're shooting long-range, your left hand should be under the butt of the weapon unless you're not on a rest or bipod, and I could see a vert grip just getting in the way of stuff.