Although manual light dep greenhouse operation can be executed surgically without fail, any deviation in your growing practices can result in lower yields and potentially lesser quality.
When it comes to getting the highest light dep yields, attention to detail is important because plants are sensitive and will adapt to conditions. In some cases, they may adapt in a way that is unfavorable (hermaphroditism, poor bud formation). If light deprivation greenhouse conditions are controlled with precision, the crop's outcomes are much more predictable. Automatic light deprivation is going to be the most consistent greenhouse light deprivation system and in most cases will result in increased yields. Remember, the greenhouse must be 100% light-proof for crops to succeed!
Another factor to consider is that manual light dep presents challenges to ventilation. If you have a greenhouse with roll-up sides, you need to roll the sides down when you pull the light deprivation curtains. If you do not roll them back up again after the sun sets, the greenhouse will usually have major issues with too much heat and humidity collecting inside. This often leads to mold, pests, and plant stress that decreases yields. You'll also have to roll the sides back down before sunrise and then roll them up when you pull back the light deprivation tarps when the dark cycle is over. That means manual light deprivation requires twice as much labor when you have a greenhouse with natural ventilation!
Automating light deprivation is an especially good option to reduce labor. If you are using mechanical ventilation it is more manageable to use manual light deprivation systems. However, if you are using roll-up sides it is much less time consuming to use automated light dep systems since you need to pull the curtains more frequently. The most reliable and least labor-intensive option with the best ventilation will be a greenhouse with automated light deprivation and automated mechanical ventilation.