I'm not defending the manufacturers. Quite the contrary; I want to empower you to inspect your products for typical errors.
Regardless of manufacturer, when I buy a lens I run a couple of quick tests to determine its working the way it should. I'm sure there's better and more elaborate ways to test lenses, but this works for me. If you have suggestions, I would like to hear them.
First I shoot a focus test. If you have never done one of these, everything you need to learn is here:It only takes a couple of minutes (after the first time).
Next I do a sharpness test. I lay a newspaper down or hang one up (usually the stock listing in the business section) and shoot it at various apertures in good light. I view the results on my PC and compare the various corners to each other. Is the left bottom as sharp as the right bottom, et cetera. I do the same with the sides and even check the center to make sure nothing seems odd. If there is a bad corner or an unbalanced lens you'll notice it quickly. And again it doesn't require much time or energy to do this test.
You should also do a quick physical inspection of the lens, making sure the glass is clear, the movement of components is proper and even jiggle the lens to make sure nothing rattles or is loose.
I usually do these tests right away while I'm still within the return period from my retailer. If I spot something that looks suspicious I return the lens and request a new one.
I recently purchased a new Sigma 24mm 1.8 and noticed it front focused. I sent it back to B&H received a second unit that tested perfectly. Sure I was bummed I had to do this but the alternative is not buying anything new or wishing on blind faith that the lens is perfect.