It's a bit hard to talk about unions in general. Eg the Americans have these unions for people working in the entertainment industry (eg actors guild or so?), and those people usually work on a project basis, don't they?

On a historical note, I am currently reading A Generation of Materialism. The book has some very interesting bits about the early history of unions in Europe.

Of course. If I think of cases where people might expect to have the same employer for life I think you'll be more likely to find unionization there. Not that it cannot happen outside of that condition.