Here is an exhaustive Canon EOS R5 review, a camera "significant and packed with special…
It’s a pretty technical article, yet it is understandable also for people not into engineering. In the conclusion Mr. Cicala writes:
Sigma has made a very nice zoom here at a very good price. It’s obviously not going to create the stir that occurred with their 35mm f/1.4 or 18-35 f/1.8 lenses, but it’s a nice addition to their lineup.
Canon shooters who have a 24-105 f/4 IS have no reason to change based on optics as long as the promotional pricing remains in effect. The Sigma and Canon 24-105s are nearly identical. Similarly the Nikon 24-120 has been available on discount lately, so I don’t see a big motivation for Nikon shooters to change, either.
Of course, if the pricing changes I wouldn’t hesitate to buy the Sigma instead of the name brands. If nothing else, I think the presence of the Sigma will keep Canon and Nikon prices reasonable.