Dark energy

You should only upgrade the most useful units to levels 1, 3, 4, and 5...

I also upgrade them, but there's still a slight drawback due to upgrading units to levels 6 and 7, isn't there?

And if you upgrade levels 6 and 7 too much, it will disrupt the balance... Upgrading to levels 6 and 7 can be removed altogether, and more energy can be given instead.

What balance are you talking about? Everything is well-balanced (even very well!), and nothing needs to be removed. If anything needs to be taken away, then the super ability of each faction should also be reduced. I don't think many people would like that!