With a properly designed system any outside force wouldn't matter. Skier weight, wind, ski, rollers, ect.
With an accurate enough GPS, fast enough steering and a tightly tuned PID loop you'd never see the boat deviate more than a predetermined amount. This is exactly how ZO works. It sees a speed deviation from the baseline speed and it reacts. How fast it reacts and how hard it reacts (the PID loop) is controlled by the ABC123 settings. ZO could have been programmed to use 100% throttle to get back to baseline ASAP, but skiers likely would have found that harsh and undesirable. I have a feeling that 'perfect' steering and 'perfect' speed are more of an ideal on paper, not in the real world.
I for one like driving the boat, it's all part of the skiing experience to me.