There's a phrase they use in the Heresy books, "transhuman shock". The idea that marines are so big, so fast etc that it's almost repulsive to mortals, even on the same team. And I think it's easy for that to get lost if they're over-used. Keep them for shock and awe sequences while focusing on "normal" humans imo. Besides, one of the best parts of the setting is the hugeness and gothic madness of the architecture and ships, and they'll need budget for that too.