While keeping your entire hand is extremely powerful, keep the ten-card hand limit in mind - if you're unable to reliably play, discard, or otherwise dispose of five cards a turn, the pyramid will eventually cause you to draw fewer cards.
It interacts particularly badly with Wounds, Curses, Statuses, and other Unplayable, non-Ethereal cards that can linger in your hand forever or even fill it entirely. However end of turn effects, like Burns and Doubt, will discard themselves at the end of the turn. Pain and Normality are particularly harmful.
This relic is particularly powerful with small cycle based decks, such as Rampage. If you can get your deck down to 10 cards after powers and exhausts are played, you can guarantee you'll see that key card every turn.
Despite having similar effect, this relic does not Retain your hand. It merely stops you from discarding cards at the end of your turn. Cards that synergize with Retain, specifically Establishment, has no additional synergy with Runic Pyramid.