Drop rates are shitty, and it goes completely off Digimon lore to make money off of that fact. Rather than being able to say, train an Agumon into a Wargreymon, you need to pull one that is even capable of becoming a Mega. Chances are that you'll just get a shit ton of Agumons that can only go up to Metal Greymons <_<
IIRC you only get up to 15 rubies for beating a story stage for the first time, so getting enough for summons takes a while
Some requirements to 3 star stages are a pain in the ass because the boss or sub boss dies too easily
YOU CAN'T FUCKING TARGET WHICH ENEMY TO ATTACK <_<