Whenever I buy and/or test lenses, I don't give a cr*p what name brand it is. I test how it performs. If it's subpar I won't buy it, or return it or send it in for repair, etc.
I've had about 30 lenses during the last 5 years, 4 different brands, but mostly Canon and Sigma. I had good Canons and bad Canons, as well as good Sigmas and bad Sigmas, Tamrons too. While the percentage of duds may be higher in certain brands, all brands have good and bad lenses. Just look at Roger Cicala's (Lensrentals) tests. He's got dozens of copies of the same lens at his business. He tested a bunch of Canon 24-70 2.8 (Mark I) lenses and the copy variation was pretty shocking.
Once I had A Tamron 200-500 tele zoom that I purchased for around $400, since it was a good deal and I'm into wildlife photography. I also had a Sigma 150-500 OS at that time.
The Tamron was a disappointment first; very soft, even with controlled, manual focus tests. Fortunately it was under warranty (thank god for 6 years), so I sent it in for realignment of the groups/elements. Came back the same. I sent it back again, came back MUCH sharper, at that time it had better IQ than my Sigma. So softness is fixable and is not necessarily a representative of a particular lens model.
Then I traded it for a Canon 17-40L.