I'm raising the BS flag on this statement. Have you ever owned one, or are you just repeating what you've heard?
Sigh... Yes, I currently own one that had a broken firing pin and was full auto until I replaced the pin, as well as two others in years past (one was a cannibalized incomplete gun, not worth fixing). I have also worked over the years on about a half dozen or so of them, two of which had cracked breech blocks, one with a broken extractor and one with a worn hammer, which wasn't really the gun's fault as the someone had tried to "adjust" the sear notch. Those are the ones that I remember specifically. The others were probably cleanings.
The guns are well made, but the design leaves a lot to be desired in terms of simplicity and sturdiness. It's been pointed out before that Pedersen had to avoid designs that were previously patented by Browning and/or Colt, and the result was, shall we say, clever, but not all that good. In other words, close, but no cigar.
While I realize that 7 or 8 guns aren't a comprehensive cross section, working on them and seeing how they were made and how they function was enough to keep me from carrying one for self defense, in spite of their thinness and excellent grip. I don't shoot the one I have now, either, because parts are drying up.
Now, having said that, I have not seen nor handled an R51, so I can't pass judgment from actual experience. The one that my LGS received went right back before I had a chance to look at it. In this case, yes, my disdain for the R51 is based on the many horror stories I have read, mostly on this forum. My opinion of the original 51 is based on my experiences with them.
As always, YMMV.
If you're satisfied, lower your BS flag, please.