Alliance are a serie of different mods. On the Star Wars GC, you can play the civil war Empire vs Rebel era + Black Sun and soon the Republic and CIS.
In the middle of this video, there is a funny space battle where a inferior group of Rebel Starships defeat with good tactics a powerful group of Imperial units. I enjoyed it a lot.
Clearly the main advantage from the Rebel Alliance, it is the fighters and bombers, they are a lot more powerful than the Imperials. But now it is more balanced because the SSD can support more damage and they are not destroyed too easy.
Other important improve is the ground B-Wing, the previous.....I did not like it too much. The normal flying position from the B-Wing is bad for ground battles. But.......it had a simple solution, the B-Wing can rotate the cockpit and there is not reason because it can not fly in a more normal position from wings. Other change is the increase from the company to two units.