They're fun, but a little weak to make a difference. And maybe for this card its for the best, as its free to use and the times it actually matters it makes a difference of 26 cards. The current EC is useable only during 2 games tops, meaning that even if the sideboard guarantees victory, the player has to win one game normally and ofc the sideboard never actually guarantees the win. It gives a new challenge for deckbuilding, unfortunately it will rarely make the difference between winner and loser. Maybe if it cost 1 relic to use and you could sideboard 3 cards per lost game permanently (for the rest of the match) like in an actual sideboard (replacing cards rather than adding), it could have been even cooler. Also it's a bit awkward that we have to reveal from which deck we sideboarded even if we havent played that deck yet.
The previous card was unfortunately way too weak to use; it should have revealed salvage/discard of target previous rounds match (so its not random guessing and all somewhat alive teams could have benefitted from it) and cost very little (either a single card per use or allow a single free use to each team), but it was a nice idea and balanced too.
