I'm on the left, am extremely pro-union (I think tech needs to organize ASAP), and I'm troubled by public sector unions. Private sector unions have different incentives than public sector unions, and there's a pretty obvious track record of abuses among public sector unions.

The AFSCME (lobbying organization for public employees) is in the top 5 for campaign contributors since 1990 and supports Democrats almost exclusively. Lots of republicans who aren't hardline ideologues oppose expending the influence of public employees' unions.