I'm sorry about your personal misadventures, but your opinions are very much in disagreement with the majority of what I've read/experienced myself - especially for some of the lenses you mentioned.
If you're happier with Canon lenses good for you, but I still think your generalizations are far from reality.
He said the 70-200 and 400 f/2.8 IS are better than the 120-300 and 100-300, which is absolutely true. Canon's telephotos and super telephotos are some of the best in the world. He said the Sigma 50mm f/1.4 and Sigma 85mm f/1.4 focus inconsistently, which is something I've heard plenty of times (with any 3rd party lens really). And he finished it off saying that the 35mm f/1.4 is the only lens superior to it's Canon counterpart (which most people agree on).
It seems like you're reading what he said very selectively since most of what he did say tends to mirror what I've read/experienced. And he's not really making "generalizations" he was very specific about the lenses and the problems that they had.
He said that all of Sigma lenses, except the new 35mm, are worse than their Canon counterpart.
Now, it doesn't seem to me that Canon has anything similar to a 120-300/2.8 or a 100-300/4, right? What's the point of comparing a 120-300 zoom with a 400mm prime (that costs 5 times
as much)? And by the way, every comment I've read on both lenses was ecstatic.
Let's compare apples to apples at least. Many people prefer their Sigma 50mm (in spite of focus quirks) and 85mm to their Canikon and even Zeiss equivalent because of the razor-like center sharpness wide-open and nice bokeh. Many people preferred the 50-500's smoother bokeh to the 100-400L. 17-70mm VS Canon 17-85mm. And so on.
Beware that I'm not stating the opposite: I'm not saying every Sigma lens is great or better than its Canon equivalent. I'm going so far as disagreeing with the opposite assertion.
Some people have a very different mind attitude towards Canon and other brands. When Canon products have problems, well, it just happens, sample variation is a hard reality and so on. When other products, most of the time much cheaper, have similar/other problems then it's all about terrible QC.