As things stand, Marauders consistently become the most militarily-powerful factions in the later game, and the major factions have a great deal of difficulty dealing with them. Given that the setup for Marauder factions, if actually examined as empires, would be extremely poor at research (being empires with no labs and with either zero or small free population), perhaps it might be a good idea to have the current tech advancement curve for Marauders be dramatically slowed or stopped so that they technologically stagnate relative to major factions. This might help the AI major factions, and leave us from wondering how it was that these isolated planets with limited mining capability and poor research setups managed to build multiple battleships with heavy railguns, primary beams, and high-grade defenses.
I'm all for Marauder worlds being too tricky to take in the early game, but I think, perhaps, barring dramatic influxes of resources and technology, that they should eventually become relatively easy prey for major factions as major factions develop scientifically and industrially. Given how badly Marauders currently bleed AI factions, this will likely have a good influence on the success of AI factions as well.