There's ample historical precedent of unions in US being anti-minorities and anti-immigration. It may be a far fetched worry in this climate (although given electoral demographics, I wouldn't be so sure), but it is a legitimate one.

Here's history:

https://en.wikipedia.org/wiki/Asiatic_Exclusion_League

Here's a recent example of unionized workers exhibiting racism:

https://www.cnn.com/2019/01/16/us/gm-toledo-racism-lawsuit/i...