Currently the GDI/Nod win ratios are definitely skewed but it depends on the maps.
On maps without defense Nod will definitely win more due because of the SBH. In Ren X, the new players aren't accustomed to defending the base so GDI is in general more careless and susceptible to nuking. In the original Ren, there would always be a Mobius guarding the entrances and an APC patrolling the base. Now, you can set a nuke and everyone on GDI will just assume someone else will take care of it. I'm sure that given time, GDI will start defending better, as the original Ren probably suffered from the same problem when it was released.
This is further exacerbated by things like quiet beacons, illegal beacon placements into a building's wall, and the glitch in Walls Flying, but this can be fixed by the devs.
On maps with defenses, GDI will win likely due to their superior vehicle power and the large nerf to the Nod artillery. I have yet to see a Field game where Nod managed to take the field.
Just keep in mind this is a work in progress and the original Renegade was not perfectly balanced either.