I haven't got much to add. Only the fact that the rate of which spirecraft and golem exo's build up (and the strenght of the exo itself), depends on the AIP. the higher the AIP, the faster they build up and the stronger they get.
So if you're someone who tries to keep it's AIP as low as possible, you'll find the Fallen Spire exo's to be tougher. But if you usually end with high AIP like me (I often hit tech III, sometimes even tech IV), then the Spirecraft and Golem exo's can be really tough.