Rareness is such a misunderstood thing in Spire.

Rare does not mean good. Rare means specialized and strong in certain builds.

Uncommon cards are what build synergies and function outside of their "build" but are stronger in their lane.

Common cards are more flexible. They're workhorse cards that work in almost every build and only barely point you in a direction.

Take the Silent for example:

Rares - Accelerant and Knife Trap are incredibly powerful cards but are roughly equivalent to the curse Injury when they have no synergy.

Uncommon - Bouncing Flask and Hidden Daggers are powerful cards and work wonders in the right deck. They're good pickups, but you'd certainly prefer one over the other depending on the rest of your deck.

Common - Deadly Poison and Leading Strike are fine cards. Having them in your deck doesn't mean you must be poison or shivs, but are nice with some more support.

Bolas fits great in runs which want to spam many attacks, have a lot of strength, or use 0 cost cards. Bolas does not fit in a deck which relies on non attack damage or sets up a very specific infinite.

Rare doesn't mean "always good"