The Tavern

If you're not undead or a follower of yredelemnul/dithmenos and use this item, will the accuracy reduction affect you?

I believe it will, yes. But you can do just fine overcoming the accuracy penalty with high Weapon skill, especially if you've got a source of Slaying and/or a highly-enchanted weapon in general.

If I had that Ring and didn't have anything clearly better, I'd totally use it. Not worshipping Dith/Yred might even be more of a reason to use it, in fact, since then I get the Umbra aura as a novelty and it's not wasted.

It's up to you.

What's more important is that the accuracy penalty also affects the monsters.

Yes

The accuracy penalty would be more relevant if not for the fact that very few enemies actually have decent EV. So, uh, I guess just be aware that that miniscule percentage of enemies exists, and handle them accordingly.