The bottom line is that there has to be some sort of quality control on safety critical parts, and this sort of audit trail costs money and if you want to fly you have to understand you must pay for this quality control.
And there is!
Any reputable manufacturer will have a QC system in place, as evidenced by the fact that steel-framed buildings, bridges, cranes, schools, hospitals.......aren't falling about around our ears!
All metric fasteners have a strength-code embossed on them (not sure about Unified, it's been a long time
)
Even cheap "knockoff" Chinese Quad "bikes" are NOT notorious for falling apart......granted, they usually rust pretty quickly and screws and bolts often shear off if they don't get pre-treatment....so, I;d say the threat of in-service failure is a red-herring....
The threat is real, of course, Statistically, it's vanishingly small, as is the chance of a piece of alloy tube or sheet in a Microlight failing......If you think I'm spouting crap, please explain why the CAA have decided that lightweight single -seaters are perfectly OK being unregulated...
AIUI, not evena basic fabric-strength-test (Betts test? ) Joe soap, the pilot of said machine , is quite within his remit to pick up nuts and bolts from his local hardware-store...Fortunately, the nature of the undertaking/hobby, and the mandatory pilot-training, make it probable that he is well-aware of the safety-implications of using sub-standard components.....He'll also know that the "off the shelf" High-tensile fastener-stock from a reputable engineer's supplier has probably sold in it's billions , against the aircraft-approved equivalent in it'sthousands or millions....and probably with a lower failure rate,despite Fred the steel-erector not having a torque-wrench or an understanding of elastic deformation, etc.
There are areas where items are stressed to the limit....IMHO, they are undersized...in these instances, I can see the point in NDT -ing every itembefore release from the manufacturer.
The Rotorway helicopter comes to mind as an example of engineering down to the margin....I believe it has now become reasonably safe and reliable.
I remember looking at a Rans aeleron pivot -bolt and bracket,thinking how flimsy it was, then realising that it was probably about 10 times bigger than it actually needed to be, for the loads developed in service.
Certified aircraft, carrying thousands of Pax, in the air ~ 20 hours a day, 7 days a week......yes, I can see the value of an extra layer of quality-assurance.
I struggle to see any benifit to GA and the statistics and evolving legislation would appear to back up my view.
Was very impressed with the Vans AD's re-cracks ...what a well-built machine, for an "amateur" product.