Legend, I have multiple missions without any Siren, Haven defense, special missions, Ambushes, Scavenging and even Citadels. It’s simple when I got my first Scylla in a Haven defense, I quoted it was only Scylla and Chirons, I had Sirens before.
Past that the only rare Sirens was in Lairs. I got a sonic Scylla which is in my opinion a quite dangerous Scylla, so there’s no logical reason for having Sirens banned from my play.
Complains of many players about sirens are absurd and only weak knowledge of the game, treat them as rare enemies is total non sense. Even pack of Sirens don’t need be avoided.