I have actually heard that the 100-400 was a poor choice to use on an APS-C camera; but had not heard that about any of the 70-200's until now.
Sample variation, missed focus, camera shake, etc, probably account for most of this. There are way too many sharp samples from these "older" lenses like the 100-400 and 70-200 f/2.8L IS mk1 to say they are soft. That cheetah pic above is a solid example of that.
OTOH, every time my friend shows me results from his 70-200 mkII, I go "WOW!" because it is clearly much improved over my mk1.
When I compared my mk1 vs a nice prime like my 300 f/4, of course the prime is better, but the zoom is still a solid performer.