Hey, thanks for the feedback!
I'm definitely looking for ways to improve the balance for the upcoming Steam release, so feedback like this is really valuable!
Right now every unit drops the same number of gems (2-3), so it's easier to maintain them if you summon mostly inexpensive units. In the full release there will be some units spawning automatically, plus also upgrades that generate more gems. I might also consider making units drop some % of their cost.
The chest traps used to be extremely overpowered compared to anything else, so I quickly nerfed them post-jam. Seems like it was too much, but I'll try to make them more useful in the full release.
The exploding zombies do some damage to your own units, but this damage is not scaled by their damage upgrades. I might make them do even less damage to allies, though I like the idea that they can chain-explode.