It's really really hard to hit the right spot on a moving target. In all my martial arts time I can only recall on witnessing less than a hand full of successful solar plexus shots, delivering one myself. Livershots were way more often and successful, but somehow they are also rarely used by UFC fighters. They tend to get tunnel vision and start to head hunt only. Maybe that will change after the spectacular liver kick finish in the Lesnar vs Overeem fight.