What are the pros and cons of well compensated STEM graduate students joining a union?
The Fair Labor Standards Act (FLSA) of 2016 set a minimum salary of $47,476 for professional employees to be exempt from paid overtime. in response to this, the NIH changed its postdoctoral salaries.
While the NIH says
NIH is fully supportive of increased pay for postdoctoral researchers and has proposed to increase the NRSA postdoctoral stipends to levels above the threshold
their behavior suggests otherwise. What they did is simple increase all postdoctoral salaries below the minimum to slightly higher than the minimum. While historically a year 1 post doc made 4% more than a year 0 postdoc, after the adjustment they made the same. There was no increase in the predoctoral salaries. In FY 2016 the NIH predoctoral salary was $23,376 so slightly less than what you are currently being paid.
While you may believe you are well compensated, some would argue that $29,000 a year is not a fair salary if you are expected to perform unpaid overtime (and what graduate student doesn't work overtime). A union can help fight for things like fair salaries.
Some of the answers already raise good points, but I would like to add something:
Although you feel that the treatment you get is good, perhaps some of your colleagues with different situations or background have issues. For example : Do you have parental leave? Or politics concerning work-family balance?
A union could help represent minorities in their revendications, if there's any. And as someone stated, all this is very dependent on the chair you currently have, which can leave/die/change his mind.
The relation between an union and the university officials does not need to be acrimonius. They can have good relations, working both in the same direction, safeguarding the rights of students.
From a practical point of view, you rarely gain anything by joining a union or other similar lobbying organization. You either join if you feel that the cause is right, or you don't if you feel otherwise. Edit: I mean joining a union should be a moral choice, not a choice based on weighing the advantages and disadvantages.
As a member, you have to pay for the lobbying the union does, but the benefits usually go to all employees. Hence it is more cost-effective to just take the benefits and let the others pay. This assumes a legal environment where the employer has no right to know whether an employee is a union member or not. If the employer has the right to know, union members can have higher salaries and better benefits, but their relationship with the employer may suffer.