I have a few Hardinge 5C's, and have a 1/64" stepped set from CDCO on my Grizzly and a 1/32" stepped set on my Clausing. I haven't used all of the collets so can't speak to all of them, but have not had any issues with the Chinese ones. The threads inside and out have been fine. The few I checked for accuracy were at 0.0002" which may have partially been my 5MT to 5C adapter.
I'm pretty much self-taught and might be missing something, but haven't noticed any difference in use between the Hardinge and the CDCO collets. I'm not a production shop so maybe the Hardinge can take more use and abuse, but I don't abuse my stuff. If I break it, I have to fix it.
I haven't used any collets from Matt, but if they're priced in the ball park of the Chinese ones from CDCO or Shars, he's probably getting them from an Asian importer also. He has a great rep for standing by his products, so you will end up with a good set if there are some issues with a few of them.
Bruce