Fulle barre chords are easy on the steel string (if you practice enough), and I don't use my thumb. As a classical guitar crossover player, you don't use you thumb for fretting...you should try to execute a full bar on the big fat ol' neck classical guitars have! Like bert says, your finger joint falls over on of the strings and buzzes, so you never really get a perfect sound out of it. It takes an amazing amount of strength and precision placing to get it to sound right--neither of which I have!

--Mbo