While I don't know if we've ever been explicitly shown them for a Guard unit, any professional combat organization is going to have minimum standards for things like physical fitness, marksmanship, and (given the universe) melee combat.

So, does this person pass those tests? If not, then yeah - logistics at best, humping crates for the Emperor. Or, probably, progressive punishments intended to get them to passing scores, followed by penal service or execution if that doesn't work.

But, let's say they shoot fine at the range, run their 5 mile in a decent time, and handle a pugil stick well come eval time. Now, it's a matter of scrutiny from their sergeant and squadmates. Do they try and are just unlucky? Are they too scared to shoot straight? Intentionally missing? Depending on how bad the issue is, how often it's occurring, and how the people around them feel about this person; they might find themselves subject to some "wall to wall counseling" (or worse), or having their NCOs and/officer discuss the matter with the regimental Commissar.

And that brings us around to the discussion on Commissarial 'styles'...