What is more important in deck building: accessibility or power?

I've noticed when looking at new card reveals that players will often dismiss a card if it is not be readily searched out. It makes sense because if you can't access the card when you need it makes it a less reliable option during gameplay.

However, conversely I've also noticed that some cards are so high impact that even without easy access—they cannot easily be retrieved from the deck or are restricted by the banlist—they still see play.

Where do you draw the line between the two?