A migrant worker is not undocumented. Many are here in a legitimate work visa and it’s heavily used in farming.

Undocumented aliens comprise several subgroups and I suppose you can say “I really meant ….”.

For example the most common illegal is a formerly legal visiting student or worker who overstays their visa. And as you said, this is an area subject to human rights violation and discrimination: an employer can treat someone as poorly as they want, when someone is subject to deportation if they lose their job