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Original Description:

Unfortunately for the 38th Company, most of the more radical experiments with Strider tactics didn’t work out. Human riders found them extremely unwieldy and rarely contributed much to their tactical performance. Astartes made much more effective riders, but sadly the Strider simply couldn’t maintain acceptable speed and agility with half its weight saddled on its back and shifting its center of gravity. The pony pilots also found the experience unnerving, as they didn’t like having to deal with the sophisticated gunnery interface of the Strider while also having to interpret verbal (or even worse, physical) commands from the riders.
Although the tests put an end to the “Iron Cavalry” concept, some of the Chaos Space Marines requested that Striders and pony pilots be made available for additional testing. Evidently they enjoyed the ride, even if they never got to use it in combat.