The main problem I can see with orbital bombardment of any kind is not practicality, but cost effectiveness. Space-to-surface weaponry needs to be heat shielded and either simple enough to no be prohibitively expensive (which would mean dumb bombs dropped from a point to compensate for wind and such throwing them off course), but easy to counter or capable of defeating counters and other impeding factors, but relatively expensive.
People on the surface really only need to get into a bunker (and make sure their anti-invasion defences are similarly protected) in order to turn the whole idea of 'softening up the defences before landing an invasion' around 180 degrees and necessitating a ground invasion to deal with anti-bombardment defences before bombardment is viable.
If you have the technology needed to make space-to-surface weapons capable of defeating all countermeasures then a much more cost effective method would be to use that technology to make your invading landing craft able to do the same.
Especially since the defending planet, if they're smart, will put their anti-invasion and surface-to-space weapons in the middle of industrial production centres and other locations that an invading enemy would want to capture as intact as possible.
The only way to defeat possible countermeasures without really expensive and delicate tech would be overwhelming force: something so hard and/or dangerous to destroy and destructive that it will render defences completely useless, but also make the entire invasion moot. Something like a swarm of nukes, a gigantic meteor or a wide area mass volley of lasers.
So most likely orbital bombardment would take a role in warfare rather similar to nukes in modern times: Used as a threat more so than an actual weapon or used when the target absolute, positively needs to be destroyed.
Also:
Destructionator. I'll admit that I made a mistake and was blatantly wrong, but here comes the kicker:
In addition having been wrong, I'm also sorta more right than I even thought and you're wrong on at least one point: it's very possible to have a recoilless magnetic weapon.
Why was I wrong?
Three year ago, I was majoring in applied sciences. For out physics projects, we used a rig fitted with an eletro-magnetic device to propel a projectile at faster than most people can run. We were given a failing grade.
I eventually switched majors, because in addition to physics we also had to do chemistry and molecular biology, both of which required us to use volumetric pipettes, for which I lacked the manual motor control (I am physically incapable of getting the right amount of liquid into those fuckers using a pipette bulb, except by chance and that meant I had repeat our practicals four or five times more than my classmates, it was too stressful).
For two years after that though, I kept referring to the rig we'd constructed as a 'rail gun' until someone pointed out we didn't use a rail. So I figured it was a coil gun instead and kept thinking of that rig as what a coil gun is.
But about that I was wrong, because it wasn't a coil gun and the projectile we used wasn't just a regular piece of ferric metal.
So why am I right after all?
Because the project we were doing was a demonstration of the Magnetic Levitation principle used in Maglev trains.
The electro-magnetic devise we used was a linear motor.
And we got a bad grade because one of us (not me, I was in charge of building the rig, because I needed credits in the area of 'setting up and performing a practical experiment' because I was falling behind in that area due to my pipette problem) miscalculated the power requirements for getting the thing moving and instead of ~10 m/s the little bastard should've 'cruised' along at about ~1 m/s.
The 'projectile' we fired was a pretty basic dipole magnet.
I go bit confused when I read that coil guns fired non-magnetic bullets. Since I was still under the assumption that what we built was a coil gun, I figured that was what went wrong. I thought we got the speed wrong because we shouldn't have used a magnet.
The kind of 'coil gun' I've been proposing:
I admit my mistake, but I find it a bit disappointing that no one noticed the words 'suspend the bullet' and pointed out to me 'I think you're referring to a Linear motor, not a coil gun'.A truly well designed Coilgun has a barrel a few mili-meters larger than the bullet and uses the magnetic field to suspend the bullet in the middle of the barrel.
At any rate, since a Linear motor uses Lorentz Force to propel the 'projectile', it's one of those things that can impart momentum onto a body without being subject to the Third Law of Motion, because it is a forcefield that possesses it's own moment.
You can look that up, it's 100% correct.
I'm still inclined to say that a real Coil Gun probably isn't subject to the Third Law of Motion the way a gunpowder weapon is, since it also uses Lorentz force, but since the bullet touches the barrel, it still imparts kinetic energy onto the weapon through friction. So yes, Recoil (though somewhat less than a gunpowder weapon).
But for a Linear Motor, I can at least prove it, having done both the math and the experiment.
All that really needs to change is the name (Linear Motion Gun?) and the ammunition.
Now I noticed that I'd been wrong all along when, in an attempt to give practical example why I was wrong, I looked up 'maglev' on Wikipedia.
However, thanks to that, I do at least have an good material suggestion for the ammunition: high purity pyrolitic carbon, tipped with either an armour piercing (Depleted Uranium?) or traditional bullet (lead) material.
It should be easy enough to make for space-faring people since one method of making it involves synthetic materials in a vacuum, of which space has plenty.
Any way, I'm sorry for kicking up such a fuss when I turned out to be mistaken, but I am happy that I now at least understand what my mistake was.