I would have thought the chances of being sucked into a part of warpspace with breathable air would be pretty low, so for guardsmen or orks it wouldn't be any worse than being spaced (i.e. still pretty damn horrific!), they wouldn't survive long enough for daemons to get them. For troops with environmental systems that allow them to survive in a vacuum like Astartes, a grimmer, more 'daemony' fate might well be is store...