No the car company will not cover you if it fails, the manuals state the driver is to monitor and control the system. Note they call it a park assist, not parking system- assist equals no liability.
They dont work with trailers (yep, its been tried already!)
Yes they do work, and for most people they work well as long as they have the room. But many people expect the systems to get a car aprked in a space smaller than they can get the car parked in themselves, and teh system cant do that. It will give an alert and cancel out if you try.