Machine harvesting can be better than hand harvesting. And the opposite is also true. Let's have a look at which options.
When is machine harvesting more suitable than hand harvesting?
When the vineyard was initially designed for mechanisation. It is also more suitable for large-scale production. Indeed, machine harvesting usually costs 1/3 of hand harvesting (there are exceptions).
Machine harvesting is also preferred when harvesting by night is required. It reduces microbial spoilage, oxidation and it preserves fresh aromas. And you do not have to pay for refrigeration, useful right? It is easier to use machines than to mobilise a whole team of workers for night harvesting.
Furthermore, when rains comes for example, and you are not sure of the exact date of harvest, machine is once again easier to mobilise than a whole team of humans, and it also goes faster.
But machine harvesting not only has advantages, otherwise, everybody would use it.
It is indeed less gentle than the soft hands of workers. Furthermore, you cannot have whole bunches with machines. How would winemakers make their Beaujolais Nouveau without humans?
Machines do have sort of brains, but they still cannot manage different grape varieties in the same plot. Different varieties means different time of ripening, and machines are lost in this situation... So they still need a skilled operator to drive them!
Machines also do have sort of legs, but they still cannot work in very steep slopes. Imagine an "enjambeur" in the Lavaux vineyards in Switzerland? Oopsi Doopsi.
And can everybody afford harvesting machines? If you rent them, they are more affordable, but that is kind of the problem, there might be a competition for rental. You can still buy one, but this represents a major initial investment.
Hand harvesting is usually preferred for premium wines. Why?
Humans can handle grapes with care, i.e put them into crates. Like this, the juice from crushed grapes will not be oxidised, which would ruin the grapes.
Pickers can be very selective on the whole bunches they pick if they are well trained (you need good grapes to work with).
Pickers can work in steep slopes and in plots with different grape varieties (for example Merlot ripens earlier than Cabernet Sauvignon, can you imagine the organisation for wineries in Bordeaux?)
And sometimes, hand harvested is just absolutely required. In which cases?
When you make wine with botrytised grapes (you need to select them carefully).
When you make wine with semi-carbonic and carbonic maceration (have you ever heard of Beaujolais Nouveau?).
When you make premium sparkling wines (you also need whole bunches).
When you work on very steep slopes and uneven land.
When you work with bush vines and not trellised system (gobelet like in Châteauneuf-du-Pape).
But hand harvesting also has its down sides:
It is very expensive to hire an entire team of skilled workers for medium to large vineyards (salary, lodging, meals...).
You have to find those skilled workers at the specific time of harvest that can change according to many factors (weather...).
And they usually can only work in daylight.