Like others have stated, power and expenses are the drawbacks regarding field use of power armor. Exoskeletons in military use would most likely be used in supplementary roles like handling large amounts of amunition or transporting equipment on foot. Even then, power is still a major concern and wpuld likely restrict the exoskeletons to on base use. Unless it's something like a diesel engine attached to a basic frame, at that point a forklift is more cost effective. Also;
>Ultramarines
>Not Imperial Fists, Salamanders, or Blood Angels