I think the main difference is in testing; alcohol disappears in days, cannabis in weeks-months.

If your question is, "can we trust Z to show up 20 days a month", an alcohol test might not be a bad one. If your candidate can't be sober for a pre-defined period of a few days, that might merit following up on.

But let's be honest, the laws requiring these tests are usually pork-barrel horseshit anyways.