Warpath is the only named Tank-alternate Autobot, and is consistently referred to as a 'kid'... he's young enough to have been made after the war started/escalated!
Megatron hardly appears in the Autobot campaign, despite having Optimus inside Kaon. He blew more of his Dark Energon power on corrupting Omega Supreme than he let on, and was too busy recuperating while Optimus was fighting back.
While some reviewers have highlighted how your NPC allies in the single player campaign tend to exhibit Artificial Stupidity, it takes on an entirely different feel when you're playing with Megatron as an AI ally. Charging head-long into overwhelming enemy firepower, shooting his own allies, or otherwise causing their deaths through his own self-absorbed arrogance is completely in-character for Megatron.