Power requirement will be huge. Unless a breakthrough in engine/generator you'll need a very high energy density storage device.
Even if a storage device exists with the same energy density as gunpowder you are still looking at converting more than half of existing space for munitions into energy storage devices, though this also means you only need the warhead part of the projectile.
Not to mention that if you double projectile speed you'll require 4 times more energy.
Weight will be an issue.
cooling will be an issue
maintenance will be an issue.
A large railgun is actually easier to field than a smaller one considering practical issues.