Morgoth890:

Okay this is to to CAB_IV, thats all true,though of course, the Covenant ships sent in pursuit of the Pillar of Autumn might not be built to have it, the Scarab might be more of a big scary, Moral Crushing Invasion Weapon.

Now Regarding this thread, the Scarab should have hit points, and it should be vulnerable. 

I hear people talking about how it should be able to destroy whole groups. You think thats true to Halo Canon? No, the Scarab , should need a balanced support groups to support it against those pesky Warthogs and AT Infantry.

Not even in  RTS's are God units supposed to function by themselves, and since Halo Wars is a Modern Game the Units should know to attack weak points by themselves. 

 Yeah, but the invasion fleet was already at Reach.   Besides, speaking of Halo 2, why would scarabs be monsterous and invincible? Even Sarges says "they're tough, but they ain't  invincible".

 The scarab should not have weak points for you to individually target.  it sounds good on paper, but in practice its a pain in the ass. as you've said, the you can assume the AI already did that.