Time to parrot what many already said:
1) The Plums (red lvl2 flies) are really annoying to deal with if you didn't get a lobber baby or the Bean. I guess they're based off of the Jetpack zombies from PvZ2, but those can still be hit by straight shots when flying above plants, and they also have more counters (Tall-nuts, Infini-nut's plant food, blovers/hurrikale, etc). The simplest way to fix them would be to let them be hit by normal shots.
2) The Hardies, Stoners and Sperm Whales have waaaaaay too much health like holy hell. Moreover, they don't care about status effect or knockback, which makes it even worse! And both hardies and sperm whales will instakill plants...
2a) I'd definitly reduce all of their respective health, they're damage sponges.
2b) For Sperm Whales, I'd make them susceptible to status effects/knockback, because they're awfully fast. Also, maybe they could drop 50 hearts when killed after eating a baby? They're so buffy it's impossible to kill them otherwise.
2c) Hardies should probably take damage when bashing against babies (similar to their TBoI incarnations, they could hits obstacles 2-3 times before dying).
3) Babies should definitely drop faster during bossfights. Either that, or you start the fight with 2-3 brother bobby in hand.