Problem with the assault cannon?
You have a choice:
Take one heavy bolter for troops, one lascannon for tanks.
Take two assault cannon for both.
That's what it boils down to- a no-brainer.
A single Assault Cannon of course isn't going to win the game for you, cause the enemy army to spontaneously combust, etc.
But it is meant to be a terrifying anti-infantry weapon- like the Minigun in Predator (the Arnie film) but slightly bigger. Being Str 6 and 4 shot represents this, as does the AP 1 on a 6- it mows down troops.
But it should not crack open a Land Raider better than a dedicated anti-tank weapon like a lascannon.
I propose the Assault Cannon having the Divine Guidance ability- on a roll to wound of 6 it is AP 1.
I'm hesitant to make all Rending weapons like this- as Tyranids especially need them for cracking open tanks. Their only other anti-tank is a Venom Cannon that can only glance and a monstrous creature that needs to slowly walk into the tank before attacking it.