I still have boxes of XSF from the very beginning. Hard and round. When seamless balls first came out they were all the same. I checked quite closely. I haven't revisited that in awhile, and I probably won't, given availability of ABS balls, which I like better.
One thing that I noticed yesterday perusing the latest list of ITTF approved balls
http://www.old.ittf.com/stories/Pictures/Balls_03_2017.pdf was how many different brands of seamless balls are now being sold. This must mean that there has been a pretty big increase in production capacity in last two years to meet that demand. How was that actually accomplished? I have no insight into that. (Increased throughput at one plant vs. increased number of manufacturing facilities). However it was done, the total number of substandard balls in existence also increased, even if those remain the same percentage of the total balls produced (and we can't exclude that with increased total production the percentage of bad balls increased). At the beginning, XSH and Palio would probably not sell those. And now, maybe, there are some companies who would be willing to put their name on them to meet the market demand, and maybe others that wouldn't. Nexy has never been shy about putting high prices on their stuff, so I doubt it means they were getting better balls. As for Yinhe, there seem to be a lot of those floating around my club these days, some of the coaches here use them (these all have blue labels). I can't see they are better than XSF.