That energy has to go somewhere, and bending sheet metal is a much nicer place for it to go rather than accelerating your body in a new direction much faster than it is designed to be accelerated.
They also sit lower, making them more stable and less likely to flip over.
Each vehicle design has its pros and cons, but yes, unibody is inherently safer than body on frame.
Now, all that said, if a Suburban and Corolla are in a head on collision, I'll take my chances in the Suburban. Other laws of physics come into play at that point.