His assaults on the Dominion were very rarely believable; General Warfield (and by extension, Mengsk) were downright clueless as to what Raynor was doing at every turn, despite Tychus being a double agent. Not only did Raynor not fail once throughout the entirety of Wings of Liberty, he also had to go up against an established and (purportedly) skilled general who had command of forces that vastly outnumbered and outgunned the Raiders -
and the Dominion had a spy within his ranks. You can't tell me that Raynor only succeeded because the Emperor let him; why would Mengsk let Tychus rampage through the streets of Korhal and allow Raynor to broadcast a message that would ruin the esteemed Emperor's reputation?