Rail guns are not fine until that God forsaken infinite range is fixed. Unless you mean the 20GJ railgun.

The blasters problem is it has pin needle accuracy, the issue you see with the rail turret is that it actually has spread fire when being fired fully automatic. I think both would be fine if the blasters had the same sort of accuracy as the rail turret, IE a little bit of spread kind of like the old halo warthog turrets from the first halo game.

Small rails are not used on ADSs because of their low utility.

They work just as well, if not better, than missiles against other vehicles, in fact, they're the best dogfighting turret due to their range and accuracy.

But they can't hit infantry for anything. Believe me, I've tried. I've hovered 5m from an infantry and still could not hit him, and considering most infantry need 2 shots to be killed compounds the issue. So they're completely anti-vehicle, reducing their overall usefulness.

So you can chose between the queen of all trades missies (being very good at anti-vehicle and infantry), lolblasters (which aren't bad, just impractical), or highly specialized rails.

There's really nothing wrong with the rails, it's just how they work. The only way to make them viable in a dropship is to use them as side gunners (with a steady pilot), give them major aim assist (no), or large splash (no).

TL;DR: Rail is a niche dropship weapon that does its job well, its just the utility of missiles as both anti-infantry and AV that make it the better choice.