My stock Onslaught uses Mjolnir, Maulers, three Dual Flak, Vulcan, and some sort of missiles. It can kill endgame fleets, but when I try the simulator rush, there are so many ships and fighter wings that enough of them flank the Onslaught. (The AI is good at a flanking from afar and coming in at all directions.) Burn drive is not so much out of frying pan and into the fire, just once burn drive ends, the swarm catches up again and I am back to getting swarmed and picked off.
It just sounds like I need to remove most of the Maulers and use more Dual Flak instead.
The change to assault chaingun probably helps. I remember Hounds blasting my Onslaught past flak range before, but they cannot do that anymore.