Its not that Rat Colony/Relentless Rats have never dipped into sacrifice synergies, but cards like Marrow-Gnawer and Karumonix, the Rat King dont incentivize you to sacrifice your rats like Verminister
Midrange Goodstuff WotC print lots of high-quality midrange cards, and these are show up big time in Cube
Actually the answer is no, a player is provided with a portion of a starter deck to allow them to take on Trainer Challenges, essentially matches against AI opponents, youll unlock more cards from your starter deck each time you defeat one until eventually earning your way to a more comprehensive and slightly more powerful version of the deck you start with by the time you beat all 7 AI opponents
With the Leyline, that's a four-card combo, which doesn't sound too troublesome, but the issue is there are multiple cards that can fill the roles of pump spell or turn one sacrifice fodder, making it more consistent than it would normally be
If it was discarded to pay a cost, that cost is still paid