The control scheme is set up to make it really, really difficult to do anything but select all your units and have them all do the same thing. Want to send all your airplanes to scout around looking for crap? Sorry, your marines will be wandering around slowly on foot chasing them. Want to have a marine start picking up supply crates, or walk through a marine-only force field? Better get ready for all your tanks to bump into walls and each other as they try to do it too.
But that was just a minor annoyance until Mission 4, where you're basically supposed to protect fragile units scattered all over the WHOLE FUCKING MAP from enemies that literally appear randomly out of nowhere, all over the same fucking map. There's a few key places where it would have been nice to stash some units as a garrison or something... except that leaving a bunch of airplanes hovering over the shuttle you want them to defend isn't enough to get them to actually shoot the lone enemy wraith that plinks said shuttle to death. No, you have to select them one at a time and order them to start firing at the wraith, or else have everyone on the map running around slowly trying to help.
Actually, there is a button to select only the units on the screen, but it doesn't really work very well -- units get deselected at the drop of a hat, and when that happens while squad A and squad B are anywhere near each other, congratulations, they've just been combined.
I can probably squeak through the mission on a second try with a crappy score... probably. And maybe the rest of the game will be better suited to the 'form big blob and wander around without any use of skill' tactic that's the only one the game allows you.