Concept - the general concept is nice, I think some better replayability might add to the game, but I'm also unable to avoid any of the larger attacks and therefore won't survive day 1-
Difficulty - As I said it's impossible for me to survive on this difficulty and it's not very fun to instantly die when a larger enemy appears. I'm also not sure about that gravity field that will drag ALL the units in and explodes it's basically instant game over without a second chance.
Controls - The input options are ok, but everything is a bit small (small hit boxes as well) what makes it hard to move single units or let them pick up stuff my clicking (instead of using the group selection all the time)
Ease of use - I think it was quite clear I only had to look up the controls, therefore a tutorial explaining/ introducing the controls/ main mechanics is always a good idea.