I agree with you that it's easy to you close/close-mid, but I actually found myself trying to use the Railgun at ranges greater than the Rocket Launcher to more varying degrees of success. Unlike the Rocket Launcher, the Railgun doesn't have a scope, so what I meant by skill gap was trying to use it a mid-range or longer. There was a very specific: charge, zoom, paint target mechanic with the railgun that I found had a higher degree of difficulty in Halo 4 compared to the rest of the weapons in the sandbox.
TL;DR--"Rock 'n Rail" matches were either insanely fun or horrendously excruciating.