The gem economy seems really stingy and weirdly prescriptive. (this is going to wander a bit into combat mechanics too, but I'm not sure how to separate those cleanly)
There are very few ways to kill things in the game, and they all use seed-plots. Seed plots are a very limited commodity, with only a set number in the level (and only digseed or... plant? being able to increase those, but those are both expensive and/or rare). So having monsters usually drop only as many gems as they have hit points feels really weird. It means it seems like the optimum play is to avoid, so that you don't waste precious gems on killing things you don't have to. Sure, sometimes they drop more, but then you can end up in situations where you can't progress because you don't have any plots to plant things you need to get past areas, and all you got was the same number of gems for a game ending conundrum.
And why do monsters who are spawned from spawners (like the purple moths that-should-actually-be-pink or the flies from the towers) not ever give gems? That seems really stingy. I'm not sure, but the only way to grind monsters for gems without spending gems seems to be poison seed. I haven't found an other way, maybe vert lasers (because they cost zero gems)? This seems strangely prescriptive, if someone wants to try and grind a spawner for gems, shouldn't you let them? Maybe make a max number of mobs a spawner can spawn?