I've never had blowback issues on an MPX. I really don't quite understand it-- I can think one (kind of two) reasons for the "MPX has bad blowback" statements. I think it was a person(s) who was sensitive to blowback and complained about it publicly. Then other folks either jumped on behind them or they confirmed the original statement being sensitive to blowback themselves. I think a decent amount has to do with ammo type as well, so some folks may be using some type of ammo I never have that doesn't play nice.
There's also the possibility that folks don't really understand what bad blowback actually is. To me, bad blowback is when you're doubled over with your hands on your eyes and tears streaming down, unable to see, after you try to man up and empty a mag. Similar to nasty, burnt, more chemical dust being thrown in your face. To folks with contacts, it feels like when you put in a contact and have an eyelash under it, and it's that uncontrollable tearing up and unable to see.
I'm not trying to insult anyone sensitive to blowback... I just don't see it with the MPX. 8" barrel, 4" barrel, semi and select fire, with half a dozen plus suppressors I've yet to really see it. It's almost kind of a weird desire to shoot one of these specific guns that people complain about, just so I can see exactly what it is.
Perhaps this weekend (if "Gonig to the range is a good thing to do for Mother's Day!" happens) I'll get the GoPro out, film in as a high of frame rate as I can, and focus right on on my scraggley face and try to get some footage of what goes on at both the ejection port and then at the charging handle. If fact... hmmm... I'm going to do some thinking and try to get a short video up on it with both my 8" semi auto and our 4" select fire.