Ariat Men's FR Hi-Vis C3 Waterproof Hooded Jacket

Is this actually waterproof, or just water-resistant?

Waterproof. It's a three-layer DRYShield shell with fully taped seams and a storm flap over the front zipper, built to hold up in steady rain rather than a passing shower — and it's breathable, so you're not soaked from the inside instead.

Does it work with a harness?

Yes. There's a covered D-ring opening so you can route fall protection through the jacket instead of over it, and the cover keeps rain out of the opening.