Artifacts that affect the entire army only work for the hero who is most developed in that skill. For example, if you have a Tactics hero with 30% defense, and you give another hero in your army a shield that provides 50% defense, the higher value (50%) will take effect, and nothing will be added together. I didn't pay attention to this before, but now I try not to combine Logistics and Tactics, as it wastes a valuable slot for a ring that provides +20% defense, which is then used for gloves that increase speed.
My favorite artifact... probably the cloak that provides protection from 3 enemy attacks... it's a great item, especially if your hero is an Order mage with Hypnosis)."
Also, a sword with a vampiric attack... it's great for a melee hero. The same goes for the flaming arrow for a ranged hero. And also an artifact that causes general bad luck, and also... and also...."