Even well-optimized coilguns have much higher mass and power consumption when it comes to small 1 g projectiles. Coilguns with high velocity also tend to have large or long barrels and slower reload times. A small weapon is harder to hit than a large weapon, making it more survivable in combat, and having a fast reload rate lets it put a lot of projectiles downrange quickly, more like a sandblaster than a traditional cannon. Since destroying the radiators to limit power output of the enemy ship is the first thing you want to do in combat after disabling lasers, having high fire rate and high velocity projectiles is more important than projectile mass and raw kinetic energy.

Coilguns are better used for ships that are more heavily armored, and the ones I have tend to fire 10 g or 100 g rounds instead of 1 g rounds like my railguns, since they do better overall in the projectile mass range. An example in my collection is the coilgun I use for the Gauss' Sky drone.

why no coil-guns, are coil-guns ineffective?