The NJO battles at Coruscant, Muscave, and Ebaq were written rather spectacularly in my opinion. Coruscant and Muscave had tens of thousands of vessels on both sides, while the Battle of Ebaq had at least several hundred ships on both sides.
Against the Corruptor, they did no real damage
The damage to the Lusankya was dealt by the freighters' high-yield proton torpedoes. Those torpedoes were meant to be mounted on a space station, so obviously a large number of them would have been enough to collapse an SSD's shields, not to mention the fact that the ISD-II Freedom was busy ripping holes in the SSD's hull along with the Valiant, which was situated at the Lusankya's blind spot.
The Corruptor had its shields brought down temporarily by Rogue Squadron, and they would have recharged in a few seconds, had the Valiant not appeared and broadsided its unshielded engines. All Rogue Squadron did afterwards was destroy the bridge. With no engines or control systems, the Alderran Graveyard did the rest of the work on the VSD.
A couple of Victory-I destroyers spread out among two dozen freighters, which probably had a dozen launchers on each,
plus two squadrons of space superiority fighters, three heavily modified TIE bombers, two ISD-IIs,
and that was probably neglected in repairs and maintenance due to Thyferra having no repair facilities. It's pretty believable if you ask me.