Assault Terminators are a kick-ass Space Marine unit. Very tough, very big, and very strong. Difference between normal Terminators, they are fighting in melee (as you alredy know, what am I talking about?)
Look, these guys can destroy everything in the game. They have Thunder Hammers, which makes melee_heavy damage, and large amounts of damage too. (Note that the Force Commander's Thunder Hammer only makes power melee) So, you can teleport them right behind a tank, and start hitting its rear armor, and it will likely die before it can respond. If there is a walker, you can just tie it up to stop it from attacking your other units, while killing it. In their other hand, they carry Storm Shields. It gives more health and makes them even more tanky than ranged Terminators, which are already rock solid.
Well, their only upgrade is Lightning Claws. Let's look at the bad side, you are losing heavy melee weapons and getting power melee weapons. The loss of the shields also means less hitpoints. But, on the other hand, you are getting the best power-melee weapon it the game! You can destroy every kind of infantry with these, and not only do they cause high damage, they also "cleave" and cause splash damage to all enemies in the Terminator's front arc when they swing. They are useing full-upgraded Nobz? Get these and see what happens. Do you know what will happen? They will eat them all in seconds!
Still, because they only fight in melee, they will always receive more damage than the normal Terminators, so for their survival, don't always use their teleport to enter combat; save it for escape instead, because it has a very long cooldown time, okay? But they are slow and this makes getting into combat with the enemy army hard. So you can should use Veil of Time for a solution to get them into combat, then anytime you want to get out, use their teleport. If you had the resources to get a Land Raider too, you can load them inside and pop them out in the middle of the fight (the Land Raider Redeemer needs to get close to flame things anyway).