I’ve read very good reports about the Tamron, but think this is the exception rather than the rule, nicely summed up by you may get a sharp Tamron, but you should get a sharp canon (or nikon equivalent).
The other factor is build quality, Ive a 24-70 than has been abused in a studio for 4 years, dropped numerous times, cleaned thousands of times with edges of clothing, leant on, sat on … you name it. But its still going strong (bar a slightly wobbly mount).
At the other end of the spectrum, was recently registering focus calibrations on a 1 series body and was shocked at how poor a Sigma 70-200 2.8 was resolving detail, even stopped down. Was quickly sold on as a result.
Quality control is not as good, but canons quality control is not that brilliant itself. I have seen soft L lenses and I have seen soft tamrons and sigmas.
Wide open the canon is marginally better, but stopped down the tamron is better. Not much in it.
I love the build of L lenses and the 70-200 is the cats whiskers, but unless you are pro I don’t see the point in spending x amount extra to have the coveted ‘L’ series lenses.
Too many people worry about having the expensive gear, when they have little understanding of composition or light.