I think there's too much power. I think the time-to-kill is much too fast. Engagements are over in a fraction of a second. Marines can churn explosives, Aliens can use abilities like the energy bar doesn't exist, simply getting a jet-pack multiplies…
I think the upgrades themselves should be worked around a bit more.
Just having more armor, or more life regen is cool.. but its also boring after a while. Simple +5 to this, -10 to that, kinda stuff doesn't have much staying power.