> Do you mean union leaders or regular members? Because if it's the latter, then you have a counterexample in every 100% unionized company that ever fired anyone
This is an interesting point. In countries with well functioning high organization such as Sweden (but unlike France) the key to the situation is the peace agreement (yes it's literally called that) between unions and employers.
Basically: unions promise to work during the whole contract period.
So when strikes happen, it's when the contracts are renegotiated, which happens e.g. every two years. If the parts can't reach a new agreement, unions go on strike.
Unions don't just strike willy nilly in the middle of the contract period. Employers actually quite like this: it gives them stability and a competitive edge over countries where production might be disrupted by a strike at any point.
Strikes that happen in the middle of a contract period are called "wild strikes" and are usually not sanctioned by unions. These are very rare, compared to e.g. Denmark and France.
The union's power is tremendous when it's actually allowed to use it. For example, it typically blockades everyone from working. Even if a non-union member would want to work, the employer usually doesn't dare letting them. And even if the employer would want to keep operating with other employers, the union usually gets help from all other unions to ensure the employer can't do anything (deliveries don't work, garbage collection doesn't happen etc).