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.