having worked in the IRS, they could easily be a lot more efficient if they weren't unionized. the IRS is easily by far the most mismanaged organization i've ever had the displeasure of working at

Both things can be true. Back in the days when I respected their perspective, many US "small government" advocates advanced arguments about efficiency, and some of them made well-intentioned attempts to implement procedural and administrative reforms. No longer! Their agenda has long since devolved towards deliberate sabotage.

That said, I'm also suspicious of public employee unions. Unions, in general, are a necessary counterweight to capital - but that incentive structure doesn't exist within the public sector. The best argument one might advance is that a public-employee union can resist sabotage (as above) of their department's work, but that's poor governmental practice (I would, quite frankly, prefer that those impairing administrative function have the consequences of their actions quickly discovered), and not what usually seems to happen in the real world.