My wife works with 14 H2As who serve only one property for 9 months a year in about the best possible situation and it is definitely true that they live in bunk houses, but for better or worse you find this everywhere in the world, even singapore[1]. In 10 years of working in the ag industry though, every place she has worked they have always been documented, whether FLC or H2A. The majority I've worked with while helping out during harvest are looking for some order and steady pay to send home while avoiding cartel chaos in Michiocan; they are all upstanding individuals, some who even had professional jobs back home, for example one is a dentist.
It is very uncommon in our experience to find undocumented workers in these regulated environments, though you will still find plenty at the even city sponsored "day laborer" centers, or working for the crews that end up selling their fruit on street corners in cities. From what we have hears, 20-30 years ago there was a much larger truly undocumented immigrant issue.
[1] https://sg.news.yahoo.com/blogs/singaporescene/hidden-slums-...