Unions are disastrous for many of the same reasons that much of my country's (USA) systems are failing in various ways, political corruption.

I still live in a 'union town' (Boston) so perhaps it is different elsewhere, but they certainly push their influence well beyond the scope of worker rights in this city.

It being discovered that you didn't vote on the union line or otherwise 'play ball' with their political or public stance will get you booted from the union. Which in some trades around here makes you unemployable unless you relocate. In other words, they use scare tactics to push their agenda.

Unions had a time and place. There were absolutely deplorable conditions of work in the US for a period of time and unions were a proper solution. Today they just lay the basis for overly cushy jobs and political corruption.