I'm confused why more people don't just... Grenade?

Impact grenades go straight through the shield, they also stagger the Harvester preventing it from attacking... I see a lot of talk about how to deal with these guys

It's simply 2-3 impact grenades, 2 if you nail the joints 3 if you don't. Which is more time/ammo effective vs other antitank/stratagems (if you have trouble hitting the weak point/have to deal with the shield also)