What is the difference between the plastic used in mainstream minifigs and the plastic used to make the popular collectable series of minifigs?
On thebrickblogger.com (February 9, 2017) there was a call to report issues with minifigures. Reading the answers I get the impression that issues with clutch-power, cracks and other problems are occurring with all sorts of minifigures, not only with the collectible series. This would mean that it is not a persistent problem with Chinese factories:

http://thebrickblogger.com/2017/02/lego-collectible-minifigures-quality-issues/

Looking at Ebay I also get the feeling that 90% of all problems will never be noticed because the owners won't open the bags!