All I've been able to find is that they are a multiverse force that is attempting to take over the multiverse, not the omniverse, and not that they have come close too it as no one knows what actually motivates them. With the Time Lords we have a very real understanding of what they can do, and unless the Combine can survive having their universes destroyed, their timelines locked and their very existence deleted from history, there's nothing they would be able to do to stop the TL from taking them. The Combine would need absolute space-time mastery to even compete, not merely the ability to travel between universes. The Forerunners and Precursors could do that casually, yet they're no more capable of taking the Time Lords.