Most of this game feels really good so far.  I like the lightning attacks and the game balance.  However, I'm a bit confused by the UI.  I can't see my max HP.  Some of the UI info is redundant, like "Attack Upgrades" appearing twice in the top right.  Damage and regen at the screen bottom is redundant since it's on your top right upgrades.  I would love to see a PC version, since browser games always tend to be laggier and don't like when you tab out of the game.  Also, the sound effects that came in on game start seemed really loud to me compared to the music.   Some info on enemy stats would be great as well, since I have no idea when I'm supposed to upgrade max HP.

Health and damage of enemies are shown at the top of your screen and your hp is shown in the upgrades. The actual health bar is the red ring around your tower.

I do agree with some of the visuals being redundant as op said. Stats need to be better visualized on the screen if they are going to be shown somewhere. If all your stats are clearly visible on the upgrades, why show them somewhere. The multipliers effecting gains should be the only visual indicator. Showing how much enemies are worth would also be super helpful. The only way to tell is that 0.5 second window after you kill one. When enemies get clustered it becomes difficulty to properly see the numbers.
I like this game, but I get bored easily, so I lost interest after beating wave 100 the first time and then seeing the next goal. It takes too long to get the upgrade for improving game speed past 2x. You are better off dumping the resources for the upgrade into raw stats. All of the permanent damage upgrades are too powerful for how cheap they are. You can just spend the first few games pumping resources into dps and scrap/charge gain to get boosted quickly. Crit Chance is also too cheap along with crit damage.
I never tried going into the next tier so I'm not sure how hard it scales.
I know games like this are hard to balance to satisfy everybody, but this one is as easy as it is time consuming.