How many boards could be safely/robustly supported via USB daisy chaining per host [PC,Pi,Etc]?

I discussed this briefly with some people when the ASICMiner USB devices were announced, and there were a couple of people complaining that in their experience connecting 15+ devices via USB usually led to problems, even with high-quality powered USB strips. I have little experience in connecting lots of USB devices so I can't really say, but this might be something worth checking out for users that are aiming to have lots of boards.