And no, it's not that they're "OP".

It's that there's no warning you're being targetted by such a powerful weapon. All the rockets have lock-on, and therefore some kind of warning so you can attempt to do something about it.

But the railgun doesn't. You can be flying along happily and suddenly get blown to pieces because you couldn't see the tell-tale glow on your blindside.

If railguns gave a similar warning, players would at least have a second or too to try and evade.