Loyalist: Good for raiding as long as the air isn't thick with bombers, fair damage and they can actually retreat. Still feel kinda weak for some reason, and they have a skill that I've never found out how to use correctly. The EMP blast on death is neat, was never a game changer for me though.
Titan: My favorite fast attacker, they're great at punching down on T2/T1 units, they're fast, good defense, the thin shield keeps them useful even if their HP is low. Microing them to run in circles to dodge fire is pretty great too.
Othuum: I've had limited but great success with these. They've always fallen short on a land assault, somewhat better with an amphibious assault because of the surprise factor. Paired with snipers/arty and shields, they make great defense units, good against T1 spam too. More like a Tank for Siege Defense, not a Siege(ing) Tank.
Harbinger: Okay for the cost, the shield is nice even though it's a "bubble" but not a bubble. Being able to destroy a base and devour everything in the area is cool, having them help build is alright too but not particularly useful by the time they roll out on the field.