I have never gotten a marine medal but I assume grunt jobs like rifleman rarely ever get medals compared to specs. A demo has a much easier time killing a queen.
As for xenos, I’m less sure. I’ve gotten jellies as a variety of castes, combat and support.
I have 40 hours as GP and two medals to show for it, and had 100 hours as vehicle crewman before it was removed and have two medals for that as well. I have 400 hours in shipside doc and not a single medal for it! I can’t remember ever seeing a doctor get a medal, actually, or a nurse. Guess it all depends on the visibility of the role.
Over 300 hours on those and I didn’t get a medal once smh. Though when I do play CMO I do try to recommend active doctors/nurses though it’s difficult since a doctor can’t really do anything much exceptional, heroic or memorable that would call for a medal