The bugs you can read from apt-listbugs during a d-u are commonly available to any/all sid system users who want to read them, and may or may not be relevant to a typical system configuration. My experience (8 years) is -- mostly non-relevant. I read them, but can't remember ever seeing one that made me abort the d-u. They mostly seem to be "corner cases" -- ancient special configurations no longer working after new package version installed, for example. That's what melmarker is referring to -- not to use this forum to repeat what anyone can read in apt-listbugs or the Debian bug tracker.
Here in this forum we are looking for actual or potential breakage of the installed and updated sid OS -- loss of xserver, desktop environment, graphics, networking, broken kernel modules, new kernels not booting, etc. In other words, breakage not yet reported, that will affect many siduction users if they run the d-u. We have (apparently) many Vbox users, so you tend to see frequent reports about Vbox problems induced by a d-u. Commonly-used utilities (browsing/scanning/printing/burning/editing) packages also get attention if a d-u wants to remove or break them.
I hope this helps clarify the question.
aaaaaand, melmarker beat me to the answer!