Take it when you want a complete, self-contained army that adapts turn by turn rather than committing to one sub-faction. It is the most collection-agnostic AdMech detachment: nothing in it demands Robots, priests, vehicles or a particular objective, so almost any box you own has a job.
The honest counter-argument is the 3-DP price. At 2,000 points it fills your whole budget, which means giving up Lords of the Forge, and community consensus around the pack's release was that a 2-DP core plus Lords is often the stronger shape. Haloscreed is the 3-DP detachment that survives that comparison best, but it is a real trade.
Sample gameplan: on turn one, pick your two most important units and choose Electromotive Energisation to push them into position, with an Overlord-equivalent carrying Cognitive Reinforcement so one brick has both Imperatives. Flip to Microactuator Bracing on the turn you expect to be shot, and to Predation Protocols on the turn you commit to a charge. Spend Eradication Protocols and Targeting Override on the key attack, Guided Retreat to disengage without losing a turn of shooting, and Analytical Divination to steal position in your opponent's Movement phase.