I fail to see why someone always claims FX has QC problems. When one puts out revolutionary guns, yes, there may be a break-in problem or two. To countere any minor problems, FX does stand behind their guns. Having said that, Umarex doesn't seem to hesitate to send out problems. I know someone will say, but that's a $300 gun compared to a $1500 gun, but that to me doesn't make much difference. It is quality product or it isn't. It's Binary.
Kind of like when you look at Consumer Reports for problems with Cars. I think they'll admit that consumer expectations skew the results. Expectations for GM, Ford, and Chrysler are lower than expectations for Toyota, Honda, Nissan. So when people see an issue, much more likely to report it and get vocal, where GM owners are much more likely to put up with and just except it. Having said that, look at the reports and you'll find that Toyota, Honda and Nissan still outscore GM and Ford on the quality scale year after year.
I think you run into the same problem with FX vs the lower end guns. Expectations are so much higher, to the point of being completely unrealistic. Then someone complains at the slightest perceived issue.
Now having said all that, I do own an Impact, a Marauder, and a Crosman 1701P, along with a Beeman P17 (yup, that $30 SSP pistol).
Design wise, the Impact is light years ahead of the Marauder, Crosman, or Beeman. Quality execution wise, they are all very good. My Impact was used when I got it, and performed flawlessly until I screwed it up. The Marauder and Crosman are flawless, and the Beeman is working to perfection (though expectations are a bit lower considering price point. Crappy trigger).
So, you can get really good execution of design at about any price range. Do your research and figure out why people are saying what thy say (I always look at the 1 star reviews to determine how many are legit or if the reviewer is an idiot. You'd be surprised at the number if idiots with stupid expectations).
But as to which has the most QC issues, Umarex by FAR is my opinion. Just look at the online Gauntlet wars. I'm told the Gauntlets are getting better but Umarex shipped boatloads of crap out to the end consumer and didn't seem to care.
Which brings up another point. If FX has a problem, they stand up and support their product. When I screwed up my Impact, they lead me through the repair procedures and helped me along the way. Techs called me back to lend assistance. Think a company like Umarex will stand behind their product? Read the reviews and find out.
So, you can see that the question you asked is really fairly complex. All manufactures have problems, just depends on what they do about the problem, support the product or hide and pretend the problem doesn't exist.
Do some research and decide what you want. Companies like FX and Crosman that stand behind warranties, or do you want to deal with someone that doesn't care?