It's not good ukemi until you can survive O Soto Makikomi where the guy who out weighs you by 100 pounds lands with his elbow right on your throat.
I actually thought I knew how to fall until my first judo class. I realized at that point that I knew how to throw myself to the ground or make myself roll.
Same thing happened to me. It is really different when someone drives you onto the mat. I almost broke a clavicle when I was first thrown by a judoka...