My earliest 226 is from 2004 and came with the plastic mainspring seat so they have been around for quite some time. I've never heard of any problems with them nor have I seen anything reported on other forums. The older style mainspring seat was metal and was held on the strut with a roll pin. That system is still used on the 239 and is a real pain to remove and to get the pin back in while keeping pressure on the mainspring.
The same goes for the plastic guide rods. They seem to work ok for Beretta, Glock and other manufacturers. That being said my latest 229 with a build date of June 2011 came with a metal guide rod.