The thing to remember with Civ games it that a) they're a game, and b) while they can be inspired by history they need not mimic it. Take any of the existing civics as an example, they're based on the theory of that particular civic, not so much what has happened in history. So to properly incorporate labor unions as a civic or even a building that allows for some benefits you need to look at what they are supposed to do regardless of what history has shown in either isolated or widespread circumstances.
If you want to follow history unions are really a wash and can be ignored. While they improve work quality and pay for the working masses they increase overhead for the companies and add a lot of legal red-tape to society. So while "Joe" makes more money because he's in a unions, he has to pay more money for everything he wants because those are also made by union workers or companies that have been affected by laws brought about by union workers. But this game isn't about what history
was is is about what history
could have been.
So what is the utopian union supposed to provide?
1. A fair salary based on the work performed (happy people)
2. Safer work conditions (reduced building unhealthyness)
3. A counter-weight to the coproate machine's influence (Medium/High civic upkeep or perhaps removing the free trade route provided by the corporation tech)
4. A boost in productivity as a result of happy, safe workers. (Small production boost, maybe 10%)
To maintain game balance all you really need are some "penalties" that keep it from becoming too good. If it's a civic part of the penalty is not receiving the bonuses from another civic in the same group. If it's a building, part of the penalty is the cost/time required to build it. In most cases that should be plenty. After all a courthouse brings lawyers with it and yet their is no penalty for letting those demons run loose in your cities
