Most complaints I hear about cop unions is that they stop cops being held accountable when they do cop shit. If police unions fought for higher wages or against mass layoffs the way other unions do, rather than trying to hide genuine misdeeds by employees, I'm sure leftists wouldn't be against them, at least not so vehemently.

There's also the fact that the police are government controlled, so in theory they're already controlled democratically, making unions a little less necessary. Of course, for this to be true the US democracy would need to be functional and not so heavily influenced by capitalist media.