For something not mobile, Orbital Mass driver, it doesn't have the HP or damage to be worth the cost.
Yeah, they cost 3.6mil M+C, have 1.8mil hp (I lost one to a single siege SS recently), and hit for 900k every 10 seconds, or 90k dps.
In comparison, a single Spire Frigate:
Costs 88k M+C, has 5mil hp, and hits for 2mil every 8 seconds, or 250k dps.
Even with the vast power difference between a FS campaign and a normal one, that's far too weak. You can rescue just the refugee ship for the four spire frigates and have no Exo's, and have 11 times the firepower, and 11 times the hp for a tenth the cost. The tradeoff? You lose range, but gain mobility.
I think those things need a buff.