No real problems in a DE army besides your typical "one eye open" rule for using desperate allies.
As far as using a Culexus, they only move 6", so most people who use them were previously putting them in an allied drop pod or rhino to get them around(Can't any more if using the new FAQs). Other than that, if you can get them to where they need to be, they're great. But getting them to where they need to be is hard, particularly when the psykers you want them to go after are often on bikes, jetbikes, or have gate of infinity to teleport away from you.