In my first Brutii M/M game, which I intended to be just a quick run through of first 5 yrs or so, to "scout" the ground, the Scipii rapidly took Sicily and broke out into Carthage, as I was crushing the Macedonians. I had early on moved up the Adriatic, to take the mines, and seen off a small Gaullish force. When the Julii besieged Mediolanum at last, I besieged Patavium, and got attacked by the main Gaul army which I destroyed to take the town. The Julii, took Mediolanum much later, and was very sluggish to move against Massilia.

If the Scipii had had an early setback, and had difficulty breaking out of Sicily I would be agreeing with your experience. As it is, though I feel the Julii are a harder faction to play well than Brutii (due to their diverse strategic interests, with natural land expansion into poorer lands), I think how well the AI factions do is partly down to luck, and where/how the rebel interventions come disrupting the faction strategy.