Power is something that is arbitrary I believe, but I dont think your comparisons are a very good example. I do believe Seshat is stronger in power than Cyprian but that comes from other considerations other than raw stats. Damage special skill output, minion generation, dispel, elemental link, and mana down resist all add to her power definition by a large margin compared to just a simple riposte from Cyprian. Not to mention you are comparing a mid levelled Seshat to an almost fully levelled 4*.
I think it’s better to use power as a simple guide, but once you get into team composition and synergy, that all goes right out the window.