I've found that no matter how few enemy groups there are, the same frequency of raids will happen, except the Storyteller will just choose between the remaining enemy groups as the source of the raid/seige.
This means that if you could somehow make all the planets factions friendly you'd still get the amount of raids, the only difference is it will always be the mechanoid faction raiding you.
This means that if you could somehow make all the planets factions friendly you'd still get the amount of raids, the only difference is it will always be the mechanoid faction raiding you.