A lot of people assume that AI plays the enemies of path of champions but in reality it’s hundreds of thousands of poros.
The range of acceptable power level is much wider in pve than pvp. Balance is still important though, because it affects the relative challenge of adventures and whether folks feel it’s worth trying out different relics, plus how powerful we’d need to make new relics in the future.
For an extreme hypothetical just to demonstrate the point, imagine if there was a relic that said “+5|+5. Game start: If i have 3 copies of this relic equipped, summon me from your deck or hand.”
This relic would be so powerful it would be silly to run any other relic currently in the game. All champions that had the relic slots for 3 copies of it should run 3 copies of it.
Furthermore. All late game adventures that are supposed to be challenging would have to be balanced assuming players can get 3 copies of this relic. That means not using this relic would likely be a guaranteed defeat on that adventure.
While there are certainly exceptions, while speaking relics can absolutely do exciting and powerful things - when a relic is the obviously best option for a large number of champions it can lead to problems.
If a relic is only extremely powerful with a single champion though, that’s much more okay. There’s still reasons to experiment with different relics on different champions in that case.