Every country has issues. To say that Unions don't work when the EU has a strong union culture, and then scapegoat other issues in random EU countries is intellectually dishonest.
Spain has insane unemployment rates because of generations of poorly thought out policies, not unions. They basically had a similar setup to Greece where tons of people don't pay taxes, and it survives heavily off tourism and produces few other things to make up for the tourism economy. Any country that generates MOST of its economy from tourism is going to have a rough time. We've seen it time and time again.
None of this proves the EU is a hell hole, and it has no inherent relation to unions.
Private unions can be essentialy seen as corporations, very much like any other firm, that sell labor. Such unions can in principle specialize, compete and offer unique advantages. Again – just like any other market firm. It's just that the US law makes that hard by setting them up for a narrow antagonistic role of fighting with labor management for goodies.
Great short explanation from Milton Friedman: https://www.youtube.com/watch?v=xzYgiOC9cj4