As Chris pointed out, a lot of guitar students give up when they get to the analogous F barre chord. Well one difference for ukulele students trying to learn a Bb barre is that there are seem to be a lot more really cheap ukes out there than cheap guitars- or at least ukes with really poor setups. High action at the nut can make playing first fret barre chords really difficult. I had someone play one of my ukes the other day, and he commented on how easy it was to play. In this case that's because I set the action at the nut to 0.020", while most ukes are set to 0.030" or even more. That slight difference of 0.010" or 0.020" can make a LOT of difference when you're learning.