8/0 and 6/0 uni thread does break sometimes but only some of the spools. on some spools you have to apply a good bit of pressure to break the thread, on the others it breaks very easy, i don't know why there is such a difference. maybe its shelf life. can't tell much about thicker threads.

i once noticed that the thread breaks too often, and found that the problem was in the bobbin holder, it had sharp points somwhere on the surface of the tube that were breaking the thread.

in all other means i do like tying with uni.