Maybe you’re really unlucky, or maybe you’re ordering online and your UPS delivery person hates you?
Personally, I currently have 20 lenses including 10 L-series lenses ranging from 11mm to 600mm (plus two MkIII TCs not counted in the 20), and I’ve previously bought and subsequently sold another ~10 lenses, I think 6 were L-series (plus the two MkII TCs also not counted). Of all those lenses, I’ve had to exchange only one as a bad copy, and that one was the only 3rd party lens I’ve ever bought (a Rokinon/Samyang 14/2.8 for astro).
So from my perspective, Canon’s QC is excellent. Lest you think it’s a question of different standards or not knowing how to discern a poor copy, I’ll say that I evaluate lenses carefully and thoroughly. Among other things, I use an enhanced ISO 12233-type chart to evaluate new lenses (the same AI-QA77 chart that Bryan/TDP uses for his lens tests), along with Reikan FoCal. Along those lines, I wrote most of the EF-M lens reviews posted on TDP. Point being, when I say my lenses are performing well, it’s because I’ve tested them rigorously.
It’s unfortunate that you’ve had so many bad lenses and the hassle of exchanging them. I said maybe you’re really unlucky, but obviously it’s also possible that I’m just really lucky. Bryan has gotten bad copies of lenses...IIRC, he tested 4 copies of the 24-70/2.8 II whereas mine was great from the start, and after seeing my results with the EF-M 18-150, he ordered another copy and re-shot his ISO 12233 tests with much improved performance. I hope my experience is more typical than yours!
Yes, I know Bryan tested 3-4 24-70 2.8L II...
That's was, in fact, one of the problematic lenses...
I talked to him many times about the problem.
I think you're very lucky if you got a very good copy at once! Especially because that lens has noticeable variations in performance (I didn't know this matter when I purchased my 24-70 L mark 1, and I got a good one! Never thought the mark 2 would be so complicated...)
BTW, mine were purchased/exchanged at BH and sent via 2 days air delivery.
I honestly don't think the FedEx/UPS guy had anything to do with my bad luck.
But, well... the REALLY BIG problem is when you're in a country where stores don't offer you the option to exchange a bad copy. Stores don't even recognize as "bad" copy unless a big problem in AF, stuck ring o something similar.
US/CA customers usually think that "everyone" has that chance to return, but sadly it's very far from reality if you live in other regions...