I have had a ronstan series 20 block fail on a trap line but it didn't make me go swimming.
I don't use adjustable trap lines (only the stopper ball, no blocks). I can see how they would help, but I think that having the traps adjusted so you are well balanced upwind is good. This does leave the crew a bit high at the back downwind, but I figure that is probably not a bad thing given how crazy I can be driving with the spin <img src="http://www.catsailor.com/forums/images/graemlins/smile.gif" alt="" /> (especially as I get more out of practice!).
Of course, fine tuning can be done based on the conditions prior to going out.