Maybe there is a FF wide angle zoom problem?
It's just really hard to design such a good lens given constraints on cost and size/weight. Canon's 16-35L II is a decent lens, far from stellar. It's a little better than the 17-40L. The Nikon 14-24/2.8 is better, but far from perfect (the D800 + 14-24 landscape sample shot I saw on Nikon's own website had pretty mushy corners).
If you want corner-to-corner sharpness in a wide or ultrawide focal length, consider the TS-E 24L II or TS-E 17L. Of course, one reason those lenses are so sharp is that, like using an EF lens on APS-C, you're using only the central 'sweet spot' of an image circle much larger than the sensor. When you get toward the maximum tilt or shift of the TS-E lenses, you lose some of the IQ benefit.
Good Evening, Sir, Dear Teacher Mr. neuroanatomist.
You are 1000% right, If we know the Ability of the Lenses that we have and Use 100% of their Ability, Yes, I have TS-E 24 II past 6 Months, And Still take time to learn to use this Awesome Canon Lens---Not 50% of Her Ability yet.
BUT, Super Sharp Wide Angle details from corners to corners, Sharper than many 24 MM. Lenses that I Have.
Have a great Weekend, Sir.