I have a suggestion:
How about making Enhanced Radiation Warheads effective against a certain type of land unit? Like infantry for example.
Math could go like this: it is dependent on armor and fortification ratings. Considering that majority of infantry's entrenchment is 3 (without engineers, with them - 6), entrenchment basically is the key to reducing your losses. Other additions like power armor also help in reducing losses.
Let's say that ERW basically quadruples the damage against infantry. So, if say that we design a AMM style missile, with 1 warhead strength after clicking the "enhanced radiation" button, you have a 400% chance to kill an infantry trooper with racial armor strength of 1 and entrenchment of 1. I guess it would essentially guarantee that you kill 4 troops per shot.
How about armor strength of 1 and entrenchment of 3? (1/3) * 400 = 133.33%. 1 soldier down, with potentially another one..?
Let's see how early Trans-Newtonian Powered Armor fares, plus some engineers entrenching: ((1/1.5) * (1/6)) * 400 = 44.44%. So, potentially, 3 missiles per strike, to get rid of 1 well armored soldier...
It doesn't seem that feasible on the first glance, but here's a catch:
You deploy a detachment of ground forces - to lure the enemy out, making them go on counter offensive (thus, losing their entrenchment bonuses) - and THEN bomb them!
"You FELL into my trap!"
I suppose my initial formula could be tweaked a little, to increase the defender's casualties, making the bombs more worthwhile, but the key is here.
As for other ground forces, the damage should be pretty minimal, kind of what we currently have now. I guess it could go to 100% or even just 50% effectiveness against any other ground force unit class (while infantry is at 400%).
This way, we give an Enhanced Radiation Warhead a somewhat niche use in orbital bombardments, rather than a completely role-play, but in game terms, useless, thing.