Trade Unions

Subject: do trade unions hinder or benefit the workings of a liberal capitalist economy?
On the face of it, the usual right-wing answer is , they hinder it. They impose additional costs on management, they disrupt production with strikes, they prevent modernisation by objecting to new technology.
One must also note, though, that the most successful economies are those that have free trade unions. "Socialist" countries (those with the word in the name, that is) do not have free unions, and they also are very backward technologically in the processes of industry. Oligarchies such as the former Soviet union countries are also not exactly roaring ahead economically.
So I wonder, then, whether the answer is the exact opposite.

Consider.
Before unions, wages were adjusted by the management at will. In times of slow trade, staff were laid off at will, wages cut, in order to keep profit margins at the required level.
But when unions came along, it became much less easy to cut wages. Unions campaigned for protection of employment rights, so sackings without compensation were limited by law.
Management had to manage better. They had to invest in better procedures to control stock and produce better goods. Everyone gains from that - the customer, the management, the workers.
Before unions, the usual rate of expansion in the economy was barely 2% p.a. Since then, the long-term growth rate has been higher and sometimes at 5% for years. One can't credit unions with this - technological change was a major driver. But unions keep management on their toes, keep them honest, they punish incompetent companies by exposing weaknesses.
Adam Smith's "invisible hand" at work, perhaps?
One other thing, though. Unions did make it more costly to employ staff, by reducing the rights of management to sack people at will. So unemployment has, I believe, settled at a higher rate than would have been the case.
Another consequence has been raising of the barriers to entry to a trade. Businesses need more capital to start up nowadays. So to enable the necessary competition to flourish, there has to be the capacity to hire economical workers at the price set by the market.
This means that there should be no minimum wage.

Conclusion: unions good. Labour legislation good. Minimum wage bad.

Update:
Realised later that it can be said that trade unions wanted to create Democratic socialism. They got deomcratic capitalism as well.