I'm in agreement on this subject, I've had a number of frustrating and damaging episodes with dodgy parts, some "genuine" and some "pattern". Problem is that genuine is often made by out-source from the previous UK parts makers to the Norton factory, where a inspector would check the parts delivery for production on the assembly line. That doesn't happen anymore. I have seen some incredibly badly made stuff which is literally must be hand crafted somewhere in India or Asia. Its imported to the UK and shipped out to British bike shops all over the world.
My strong recommendations from personal experience are:
Head gaskets - double check the bolt hole alignment and symmetry. The best brand so far is Viktor Reinz from Germany, unbranded stuff is suspect in material strength even if the holes line up.
Wiring harness - usually no brand, but on 3 harnesses, the fuse holder end terminal fell off! a tiny terminal with microscopic solder. Cut off and fit a solid copper or brass terminal and solder it yourself.
Carburettors - replica Amal concentrics, brand new Burlen brand had swarf trapped in the jet holder and stuffed pilot bush and way out float levels. To Burlens credit, they recognised this quality issue and sorted it out and sent me 2 new 932 Concentrics. These are a major improvement, seems they are using new alloy, much better quality all round but still the float levels where out and need re-setting. I think you will find that Burlen have really gone to some effort to improve the Amal.
Alternators - the Sparx brand rotor centre hole was .0015" undersize on a perfectly dimensioned crank end shaft. Sparx advise was "they are all like this, just belt it on". This required lathe bore machining to allow a proper zero press fit by hand as they should be. Sparx advice would have stressed the rotor excessively, possibly fracturing it internally.
Oil Filters - never ever buy a no brand oil filter.
Petrol Taps - no brand patterns, most use a rubber sleeve which softens, twists and blocks the outlet hole. Less gas at high speed, hot day and bang goes a piston. Only use t the solid brass to brass internal type.
Panel parts - pattern tailights that don't line up on the mudguard and connectors that would be struggling to last in a flashlight.
Morale of the story is as there is no more Norton factory to measure and accept quality control, its left up to the end user. If the parts vendors can get it made cheaper where possibly expertise and quality is less understood, then that will reflect in the durability and peformance of the part in operation and your enjoyment of the bike.
Mick