They’re not. It’s better to be disk shaped for combat because you present a minimal cross section to an enemy “in front” of you. If you are the attacker, you can chose your attack to face the side of the enemy ship and minimize your own damage / maximize theirs.
A cube is not stealthy and equally easy to hit from all sides. It’s weird that the Borg use it because they don’t think up their own ideas; they just copy. And no other species uses cube shaped ships. This is a minor plot hole.