One clause: friendly BLADES FOR HIRE and KABAL units' ranged attacks have [SUSTAINED HITS 1: non-MONSTER/VEHICLE].
Sustained Hits 1 means every unmodified Hit roll of 6 scores an extra hit. Across a ten-model Kabalite Warriors squad with splinter rifles, that is a meaningful volume increase every single shooting phase, for free, with no setup and no decision to get wrong.
Two details are easy to miss. First, the rule names BLADES FOR HIRE as well as KABAL, so it also covers your mercenaries: Scourges, Incubi, Mandrakes and Drazhar are Blades for Hire units, which is a much wider net than the detachment's name suggests. Second, the Monster and Vehicle restriction means none of this helps you crack armour, so dedicated anti-tank stays a separate job.
The pack states in the rule text itself that this detachment has the KABAL tag and cannot be taken with another KABAL detachment.