Don't discount the power of heroism and being amongst the "good guys". In Star Wars, that goes a long way - against all odds (and "realism"), the Galactic Empire fell largely because of it - and a lot of luck/destiny -, despite the fact that the Empire could have crushed its foe quite handily if it really committed itself to it. In that sense, it's not unreasonable that a unit of Rebel soldiers could stand up to a unit of Stormtroopers. A Rebel player just needs to remember that their equal-ish or somewhat better infantry are not going to hold up against the more numerous and better reinforced enemy. This is quite in line with the SW universe as portrayed in the movies, books, and other sources. I will also note that one can assume the Alliance has not only trained their own troops as much as possible (likely with first-hand experience), but has gone to lengths to recruit soldiers already skilled in the ways of war, which may be another way to explain why they fight on more or less equal terms.