Yeah, they're right about the camera. Mouse movement will take over the camera, but once the mouse isn't moving and you start moving again the camera goes back to auto-following. That's in vehicles at least. In third-person walking you can do either way, depending on how you set up stuff in the control settings (depending on if you set it to Joystick or mouse+keyboard).
And as far as the game being impossible to complete on PC, no. The third-person shooter elements are easier with a mouse to use for shooting. The driving elements are a bit harder IF you use the keyboard. The flying can be a pain with a keyboard, but I know people who've beaten the game with keyboard only fine.
Personally, what I did was make a USB adapter for an XBox controller to use to drive with. So I drive/fly with a controller and do ground stuff with a keyboard and mouse. Also with the tank, firetruck, and other stuff that has a turret I find it a bit easier to use the mouse with cause the turret is controlled by mouselook.