guy #1: hey dude I'm gonna get a bong...can they break? guy#2: Yea I heard if you drop glass it breaks guy#1: fuck well I can't buy this item anymore.
All bongs are breakable. Some bongs are sturdier and stronger than others. If you handle your piece with care it shouldn't break, however if you're not smart and leave it on a high ledge then it very well could break... But to answer the question at hand, I had a zong for a long time, and never had an issue with its durability. However due to the kinks in the glass it is in my opinion more prone to breaking as apose to a straight tube. I dont think though that this should affect your decision because zongs are fun to hit.
It was in the back of my head for a really long time to post a picture of the zong and I'm finally doing it. It's not a brand name zong, just china glass. I'm selling it to a friend for lik $25 because I'm getting a headford inline bub in a few days