I have the 6d with the 24-105mm f4 L and have found the sharpness to be fine generally and normally if it is off its me not the equipment.
At my day job we rent high performance cinematography lenses and I use the same test methods we use to check my Canon lenses so I can do mirco adjustments if needed (although this works best for very fast lenses). Ive only ever had one lens out from Canon a EF-S 15-85mm I use on the 7d. It was quite a lot out so in the end I returned it to Canon for adjustment now works fine.
Canon 5DS, Canon 6D, 16-35 f4L IS USM, 17-40 f4L USM, 28 f2.8, 24-70mm f4L IS USM, 24-105 f4L IS USM, 100mm f2.8L IS USM, 70-200 f2.8L IS USM II, 70-300 f4-5.6 IS USM, 50 f1.8 STM, 100-400mm f4.5-5.6L IS USM II, 1.4EX III, EOS 760D, EF-S 10-18mm f4.5-5.6 IS STM, EF-S 18-55mm f3.5-5.6 STM