Farilor and Standard are an interesting unit attachment, because they transform the feel of the Legionnaires a lot. At 6 points the Legionnaires are a good throw away unit and can get in the way of incoming models and feed the Spawning Vessel when they die. At 3 points for the Unit Attachment they become an 9 point unit that you rather expect to get some work done.
This means I only take Farilor in certain lists. Generally ones where there is some way to stack the durability he provides the Legionnaires with something else. When stacked with Epic Thagrosh's Dragons Blood for example, the Legionnaires are a solid ARM 18 all the time, and an impressive ARM 22 and immune to being made Stationary or knocked down when they use Iron Zeal. Otherwise I don't bother with Farilor and let the Legionnaires feed the Vessel.