CHALLENGE The_ CHAMPIONS LEAGUE, The_'s CHALLENGE CHAMPIONS LEAGUE

Are there any other artifacts that provide benefits not explicitly stated in their descriptions?

Well, regarding vague descriptions, it's worth noting that artifacts that grant a percentage bonus to ranged attack don't work without the corresponding ranged attack skill, that the "Spirit of Despair" artifact only nullifies morale if it's positive, and that all diplomacy artifacts reduce the ransom price by 10%. Also, artifacts that grant movement bonuses on land or at sea don't explicitly state a bonus to movement points (MP), but I don't remember it offhand; you can check in the physics engine.
That's about it, if you don't count the numerous nuances that aren't detailed for artifacts like the "Medal of Vulnerability."
Creatures are much more diverse in this regard. :)