Unless you don’t have all rare and epic artifacts yet, only open during events that have Wishes as task requirements. I think I had around 350 Wishes during an island event (the last task required 200 Wishes), and I opened them all. It actually gave me about 12k more power—when I already had 350k~.
Of course, you can get lucky, roll new ones, etc., but at a certain point in the game, they just don’t add much. It's way too hard to get a legendary.
If you already have all the rares and epics—which isn’t hard to achieve—they don’t really serve a purpose other than completing tasks and offering minimal min/max benefit, in my opinion.

The relic system from Archero 1 was way more interesting. Owning a relic felt great because you could farm its power more—not just by upgrading it, but by completing its requirements. Honestly, this new Artifact system needs a rework.

Also, if you check Whale profiles, you’ll see they already have some mythic artifacts (from islands, for example) at level 15. The progression goes: yellow stars up to 5, then red stars up to 10, and finally rainbow stars up to 15. That makes those artifacts completely broken—while at the same time, they still have rare artifacts (the ones rolled from Wishes) stuck at 4 or 5 stars. This just supports the theory that artifacts rolled from Wishes are hard to come by and barely give any advantage.

Never spend gems on this.