Try playing in a round with me, my antitank rocket gun will soon make short work of your tank
You're right about the cannons, I see very few people bothering to use them and yet they are pretty useful against the vehicles. Not the fault of the game that though (nor indeed the lack of people using the Assault antitank gun), rather the fault of the players. But in time, as people learn, that will change.
Tbh, When I first started playing I thought they were just part of the map not able to actually play with them.