The other reason to not overload on learn cards is there are diminishing returns casting Lessons
Attacking creatures with equal (or higher) power to the blocking creatures defense will cause the blocking creature to die
For example, the Giants Unleashed mission wants you to get Regigigas, Regirock, Regice and Registeel while the Battle with Champion Cynthia mission requires Cynthia, Garchomp and two Spiritomb
To increase the chances of drawing key cards during play its most common to have exactly 60 cards and run 4 copies of important cards
Heres an example of what a mobile board state looks like: Lands go left, artifacts and enchantments go right
Theres also Giant Opportunity, a green sorcery that creates a 7/7 green giant if you sacrifice two Foods, essentially simulating what happens if you eat two magic beans