The gallon sized ones I have are just jugs with a typical thread and the little finger loop. The mouths of mine are quite thin glass. If you got a blockage during fermentation, I'm sure the bottle could easily blow apart. The only other thing I can think of is that a stopper that small has little give, and with some weight behind it, could probably break the bottle also. I have fermented juice in mine with a typical blow off tube, then stopper with airlock. I wouldn't hesitate to use them again, but I am careful of the delicate mouth