For Yshtola, it came down to a mix of popularity of the character as well as being a spellcaster, her story and character development offered a wide range of abilities and moments to pull from to lead the deck
Opposing +1/+1 Counters decks suddenly grow at half speed, making their payoff Creatures feel painfully underwhelming next to yours
Then this drops on the second turn and then will last as long until killed by a foe, and if you went first, then Arcane Signets and Mind Stones are getting tapped or you get a free 1/1 and then on the third turn you get Manalith, and then don't forget this is dorks too, so everything from Reclamation Sage to Sakura-Tribe Elder to Mulldrifter will ETB tapped for blocking purposes unless you get a free Vampire
Alphinaud Leveilleur* 4 Mana for Alphinaud is a lot, given what else you can get in card draw there, and the fact its only on your second spell each turn combined with a poor size make this a creature you probably leave out of your party, unless you are wanting to make use of its partner
