Risk of Rain Enigma Acrid

Enigma is a bit of a weird Artifact. It only has two effects: firstly, it shoves the Enigma Use Item into your Use Item slot right at the beginning. Secondly, it makes it so almost all possible Use Item spawns are overwritten with a 'Small Enigma' item (Only seen if Enigma is on) that slightly lowers Use Item cooldowns. (5% per, according to the wiki)

The Enigma Use Item itself will, when used, call a random Use Item's effect, and then undergo the appropriate cooldown. You don't even have to have the relevant item unlocked!

Counter-intuitively, Enigma is nearly a straight upgrade over not having it running. Sure, you can't plan around your Use Item use, and this can lead to the occasional annoying situation; in this video I end up not managing to collect a Gold Chest drop before it vanished because I actually didn't even realize the Explorer's Key had been called initially. But overall it means that instead of Use Item drops timing/rate being 'you want one as soon as possible, but anything after that is a waste unless you don't like your first one or were happy with it but want to move on to another for some reason' instead you're more powerful from the start (Use Item slot is already filled) and every single time a Use Item drop triggers you get a concrete benefit.

I don't find Enigma very interesting, personally, but I don't particularly dislike it either.

It's worth noting that some spawns bypass Enigma, though. You can potentially get a Nematocyte Nozzle off of Young Vagrants (And possibly Wandering Vagrants? The wiki doesn't specify, and I've not seen it happen yet but I've only gotten the drop like four times in total across all my play so far) and the 'weapon chests' in the armory on the final level have a special drop table that Enigma doesn't affect that includes Mines and Disposable Missile Launchers. Enigma doesn't prevent you from dropping the Enigma Use Item, it just tries to arrange for you to have no alternatives. So you can easily end up with a Nematocyte Nozzle backed by a bunch of Small Enigmas so that you're firing it off with less than ten seconds between each shot.

Honestly, I think Enigma would've been more interesting if you'd gotten to know what your next Use Item was. Cycling through to try to get the right thing is already made a dubious choice by Risk of Rain's inherent time scaling of difficulty, and it would make the whole thing a bit more strategic while retaining the way the randomness limits your ability to plan.

It's not the worst Artifact in practical terms, but it's one of the more disappointing just because that minor change would make it a lot more interesting. Other Artifacts might frustrate me more in their actual execution, but not while leaving me feeling like it would be easy to tweak them into something similar yet a lot better.