This post is going to talk about the instances where card rarities don’t make sense between cards with similar or the exact same effect. There are multiple instances of this throughout the game. This post is going to use the more extreme example of Testudo Formation vs The Crown Jewels.
Testudo is a limited epic that’s a 3/27 with the return “your cards wherever they are, gain +48
and cost +3
this round” and Crown Jewels is an Ultra Fusion that’s a 6/50 with the return “your History cards, wherever they are, gain +30
and cost +3
for 3 turns”
Testudo gives an energy:power ratio of 1:16 while Crown Jewels is only 1:10. Testudo is cheaper at 3 energy to Crown Jewels 6 energy. Testudo is a limited Epic, easier to obtain than an Ultra Fusion (typically). Testudo affects all your cards while Crown Jewels only affects History.
The only thing Crown Jewels has over Testudo is the duration of 3 turns to that of a round (2 turns at most) So Testudo is just better than Crown Jewels in all ways but 1.
If you are going to have cards with similar or exact same effects, than keep the rarities similar to make sense. Like how Shrew and Horseshoe Bat are literally the exact same rarity with slight variation in cost, power, and ability length.

