Actually, the real advantage of the Nebulon-B series was something that you can't notice unless you look at the stats.
Before the Neb-B, the main Imperial ships-of-the-line were VSDs, ISDs, and the occasional carrier or galleon left over from the Old Republic.
Thing is, Star Destroyer-type vessels are made for ship-to-ship action; their guns are turbolasers
. Sure they can swat an X-wing down with one hit, but they're energy-intensive, slow to recharge, and not particularly accurate against fast-moving targets.
The Nebulon-B (and the *shudder*
Lancer) are designed with smaller guns speciffically designed to be able to track and lead small targets like snubfighters.
This is one of those things TG fudged to make XWA more game-friendly; I lean more toward the old RPG sourcebooks.
"Don't f_ck with the Jedi Master, son." --Mark Hamill