These things can be defended though, as impossible is some might sound.
For Ornithopter Strikes and the Saboteur, Missile Turrets become the deciding factor. If at all possible surround your base with walls, though of course leave a few open points. Dot the walls with both kinds of towers since both have a use. The walls will at least stop Saboteurs.
As for the Ornithopter Strike you might as will grin and bear it. No matter how many Missile Turrets you have, the Ornithopter Strike will hit something. Just repair it, kill the Ornithopters, and continue the game. Remember that no one ever won a battle with strictly air forces. Because you still have to take the real-estate.
As for the Death Hand, well... since it can't be destroyed just get as many units out of the area as possible. Look for spots you believe your enemy will target and keep units clear of it.
Devastators are probably the hardest thing to defend against, but it's not impossible. Turrets of both kinds help, but particularly use infantry. Infantry you say? Gronox you must be nuts! In fact, put infantry (both kinds, mind you) on the rock-outcroppings that vehicles can't drive on. The Devastator shells do little damage to infantry, making them fairly easy targets.
Other then those few bits of wisdom pretty much stick to your mid-game plan. There is no weapon in your arsenal that is totally useless. Even the Trike (as hard as that is to believe) can be used to hit Saboteurs before they are anywhere near your base.