What is the difference between the plastic used in mainstream minifigs and the plastic used to make the popular collectable series of minifigs? What is the difference (if there even is a difference) between the plastic used in mainstream minifigs and the plastic used in the popular collectable minifig series?

It seems to me that the collectable minifigs are a bit more brittle than their average brethren.
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!