They’re not weak.
The way the QA1 K-frame is located isn’t as precise, that is true. Rather than just bolting it up like a stock K you do have to take care to make sure it’s centered because the K to frame mounts have more play. Or room for adjustment, depending on your point of view.
It can make for an extra step when you’re installing your engine or headers, or doing your final wheel alignment only to find you can’t get the numbers you want on one side because the K is shifted slightly to one side or the other. So it can add some extra drama to your build if you don’t get it centered the first time.
As for why, plenty of those 50-ish year old original K members aren’t straight anymore. Or have rusted, or have been damaged, bent or broken. And the QA1 does give some additional clearance vs. a stock K, which can be useful.
I run reinforced stock K’s on my cars, and I do like them once I’ve added some welds and gussets. A buddy of mine has a QA1 K though and it’s a quality part. It just have to make sure they’re centered up. If I needed a K-frame for some reason I’d buy a QA1 vs. trying to find an original and hoping I got one that wasn’t bent or damaged, and then doing all the work to clean it up, seam weld and gusset it.
