I'd disagree, the MPX in Gen 2 had lots of issues and updates. Broken firing pins and firing pin springs in early Gen 2's that required a new bolt/firing pin design, several chamber ramp iterations to improve feeding. They had to change the barrel retaining bolts from individual bolts to a 1 piece design, because people were stripping the upper receiver recessions for them (factory loctite required too much torque to break them free and rounded out the flats in the upper that held the bolts). They had to change the bolt cam pin to the new "bowtie" version because it was badly gouging uppers (to be fair some of those were probably users putting it in backwards, though I'd make the case that a bolt cam pin that can't be installed either way like basically every AR is asking for user issues). They had gas issues and had to go from a variable gas tappet to a fixed open plug. They had extractor issues, with early Gen 2's coming with an o-ring, later versions did not, people report both working well, some found extraction issues were due to the extractor spring being installed upside down. They changed the design of the recoil spring plate due to failures. They also had lots of issues with the extractors coming loose, and also cam plates.
That's just the things I know about.....
I'd be willing to bet that all of these "Gen 2" changes were one reason why especially early in the Gen 2 MPX lots of people were having tons of issues, while others were having none. Sig took so much bad press for bringing out the Gen 2 less than a year after the Gen 1, they learned their lesson, now they won't attach "Gen X" to most guns.