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.
Get up from that and you have good ukemi.
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. It took me a few years to learn how to fall. Never thought it made me tough, but new people think it makes you insane. When I teach falls, I show them on the ground, from the knees, etc. Then I show them their ultimate goal, I get the sensei or someone else to throw me with harai or some other huge throw as hard as possible makikomi. I actually had a few people just decide then and there that judo wasn't for them.
I went the other way - 8 years of judo, 13 years away, started Aikido. Now 15 years of Aikido. Find that pure Aikido people complain when I throw because (despite not intending) there's still a bit of "finish" in my throwing. Also find that when I bump into younger Aiki folk who have a judo background, the throws have a bit more umph to them and the attention level has to go up a notch or two...