Cough, some units aren't that useful.
My favorites are the cruiser, upgrade those when you can imho. After that, the bomber is a good unit to crack shields with, but they can ONLY be used against shields with no defense against them, or you just loose em all. If you can't take the shield down with bombers, high level cruisers are your only hope, that or some of the specialist units, that I rarly see, I think one shoots thru shields.