In the following Mediator implementation, what is the main issue?

In the following Mediator implementation, what is the main issue?

class Mediator {
  notify(sender, event) {
    if (event === 'start') {
      sender.start();
    } else if (event === 'stop') {
      sender.stop();
    }
  }
}

class Component {
  start() { console.log('Started'); }
  stop() { console.log('Stopped'); }
}

const mediator = new Mediator();
const comp = new Component();
mediator.notify(comp, 'start');