Well, there are no binding contracts here. You can leave if the situation is not good and you should register your complaints with the national Wwoof organization. I saw on my Wwoof independents site a list of hosts that have been suspended for complaints pending investigation.

I think you are right, but many bad situations are probably avoided by asking numerous detailed questions. There's also lots of good advice on the sites about how to avoid dire situations including making emergency travel arrangements, getting the proper insurance, finding out the necessary travel and visa requirements.

One thing that supposedly comes up often is what to say to customs as you enter a country. If you say you are volunteering on a farm it has made some customs officers suspicious enough to bar a person from entering. You're supposed to say that you're on holiday basically. I know I'll be saying that when I'm arriving in Copenhagen in March (because essentially I am).