Some of the bosses are ♥♥♥♥♥♥♥♥ (looking at you, Aquarhythm stage 2 boss), 1 star fights hit harder than the previous stage boss at times and you can get screwed over by RNG, but I think there's still a compelling aspect to this game. The feeling of triumphing over unfair odds is the main reason that keeps me playing at least. Buy during a sale if you like looking at cute anime + a slightly unusual card game with unique-ish mechanics.

Some tips I read and applied:
Whenever you play a bullet, you draw a card as well, so having a deck of 30 cards isn't so bad. It helps that the max deck size limit is 30, so you can replace your weaker cards with stronger cards if you get them. Go for 1star fights to pick up powerful cards, then go for events/shops/campfires once you feel your deck is strong enough. No need to take every fight.
High cost "bullets" are good against single bosses since you can play them at huge discounts near the boss' turn
It might be worth it to concede and lose a few games early on purpose, since losing unlocks more cards.