There are lots of examples of the variability in AF. Each generation of AF technology gets better - and its far better than manual focus for active photography.
You have to know how to use the AF technology. Pick a portion of the scene that provides a good focus target and if possible focus on that target. A lot of photographers use the back button AF-On approach to focus over the shutter since it is easier to focus and recompose.
Low light makes AF success lower and makes it more likely to have missed focus. If you are testing, you want excellent light and neutral color temperature. With some of the testing software like FoCal, there are specific lighting conditions needed for accurate results.
Often the camera is close on the first frame but the second or third frame in a burst has sharper focus. The camera seems to fine tune focus in successive images.
Virtually no lens is at its sharpest wide open. Wide open is simply to create a shallow DOF. Its good for AF Fine tuning, but not to get the sharpest image. Usually sharper images come from stopping down 1 or more stops from wide open even on the best of lenses.
The center sensor is usually the best for AF. Cross sensors provide better and more accurate AF than regular sensors. The functioning AF sensors vary with aperture. The best AF performance is on lenses that are f/4 or lower wide open. Variable aperture lenses tend to have more AF challenges at long focal lengths.