Private employee unions are perfectly fine conceptually. Management and labor have conflicting interests, so they use the union to negotiate fair terms of employment (at least theoretically).

My issue is with public employee unions. With public employee unions, they often put in power the same politicians they are supposedly negotiating against so their interests are often aligned. And once they get their politicians in place, the terms of employment frequently become ridiculous and it is the citizens who get the raw deal because they have to pay for all the benefits with their tax dollars.