The lack of mobility, evasion, or range, and the hard cap on the number you can have per system, means that platforms should honestly be *more* cost-effective than ships. Why the heck does mounting thrusters and a hyperdrive on a hull make it, in effect, cheaper than an equally-strong stationary platform? That's just bizarre.
Comparing the starting auto-design platform to the starting auto-design corvette:
A platform costs 2.5 times what a corvette does, but offers: 3.33 times the hull, 7.5 times the amour, 3.75 times the shielding, and about 3.5 times the damage per cycle.
Comparing my current game, based on what the auto-design would offer me:
Platform costs 1.5 what a destroyer does, offers 1.4 times the hull, 2.5 times the armour, 2.5 times the shields, and 2.13 times the damage.
Cruiser costs 1.6 times what a platform does, offers 1.8 times the hull, 1.3 times the armour, even on shields, and 1.6 times the damage. This is *close* to even - damage ratio is equivalent to the cost ratio, I'm losing a bit of hull on the platform but gaining a bit of amour and a fair bit more on shields.
Don't have battleships in that game yet, so I can't run the numbers for that one.
I'll note that I didn't factor in evasion (which the defense station has none of) or range; but otherwise, at least on paper I *am* getting more power per alloy out of my stationary platforms than I would from mobile ships on the auto-designs.
Now, whether the actual performance per-alloy matches what the numbers themselves imply... I'm actually not sure how to test that without interference from the station itself.