All gun-style launches have the problem that they have to do the entire acceleration in a very short amount of time, which means very high g-forces. Also happens at the ground, so needs even more speed to compensate for loss in the atmosphere. For small & robust payloads it might be an option, but for large satellites? E.g. Gerald Bull did high-atmosphere studies using payloads fired from converted artillery guns in the 60s.
Not sure if railguns in particular add much over chemical guns that justifies the complexity.
I knew a futurist who proposed using a railgun on Kilimanjaro as stage 1 of a rocket launch system. A lot of fuel is burned just trying to shove a lot of fuel through the lower atmosphere.
But as you say, a human-rated railgun has a much lower acceleration. And there's the problem of what happens when you hit the atmosphere at the end of the gun...
Not sure if railguns in particular add much over chemical guns that justifies the complexity.