I've shot a lot of enemies, both human and non-human (though no undead), using poison of slow and it doesn't seem to make any difference in the enemy's speed. Or is it too subtle to notice?

It most definitely worked for me. It was one of my favorite poisons on my archer character. The enemies slow down to a crawl, almost like with Slow Time shout.

No you didn't read the fine print. It doesn't mean Potion of Slow, physically. It's making your enemies slow in the head! That's why it's so hard to determine if it's working because it doesn't take a genius to figure out that big sharp object goes in badman.

Yes, it works great for me. Makes child's play of killing giants.

Monsters glow red when afflicted with the slow effect, so if you don't see it you either missed the shot or they possibly resisted it.

Hmm. My slow potions say, "Target moves at 50% speed for X seconds."

Well, everyone of my dozen or so tries have either been resisted or missed. I'm going to try poisoning a mace to make sure it takes effect! (Have only used it on bows.)

Some creatures are immune to poison btw, like draugrs for instance.

I've not noticed a massive difference in enemy speed after using poison of slow either so I just started selling them since they're worth a decent amount of septims.