Speaking only for myself here, I feel more free when I have less organizational hierarchy (read: bosses) to deal with. Might be different for others.

I know of no union that would give me the flexibility I currently enjoy--flexibility to bill hourly on some projects, lump sum on others. Flexibility to invest my retirement savings as I wish. Flexibility to work 12 hours days 7 days a week for 6 weeks, then take two months off to travel. Flexibility to tell clients "this is how _I_ do it, full stop. Flexibility to choose my own wage and benefits and career trajectory. Flexibility to work for equity alone, or barter, when it makes sense.

That's my reality, and I have yet to see a union that offers anything close.