So first, I'm talking only about matches with strong players (at least Lft+ rooms) rather than the general population, since in a random Shuttle Bay match you can win with any class due to a big skill discrepancy between you and other players. With that said:
It's simply not good in any map except Space Station. In any other map: you get hunted and killed and have little to no effect on your team's ability to cap. You're too easy a target. You can't snipe past shocker domes. You can't do anything to Blasters or Tanks.
Now I get that people like Panda who have practiced rail gun a lot, may think that it is balanced. But it's important to note that he invested maybe over a thousand hours into practicing with rail gun, for it to become viable. For most players, even experienced G1+s, rail gun is not worth the effort outside of Space Station.
I think most people will agree I am a strong sniper. But I feel very powerless sniping in most Lft+ matches where everyone can just sneak up on you. I have to focus 10 times as hard while sniping with Gunner just to not die all the time to people hunting me, and I am not nearly as effective in helping my team win as if I was playing Scout or Blazer.
Additionally, I am frustrated that Rail Gun takes 4-6 shots to kill a Blaster (I will not even talk about how retarded it is to try to kill a Tank with sniping). In most shooters a snipe to the head is an instant kill. At worst 2 shots. In this game you have to snipe someone over and over and over. It doesn't make sense, especially considering that most of the maps are not designed to have good sniping spots.