Like any kind of protest or political action they work often enough to get push back from those in power but are not a guarantee. Their scope also needs to be proportional to what they are boycotting, so the larger the company or political power the more coordination and number of people it takes to get results.

Last year there was a boycott of Sabra, which makes hummus, that successfully changed the ownership from half Pepsi Co and an Israeli company to get the Israeli company to divest. It had a clear goal and a single type of product that was easy for people to understand and get behind the boycott. If someone wanted to boycott Pepsi Co, which not only makes soda but also owns a ton of restaurants and other things, it would be extremely difficult as they could weather the losses in one area due to the scope of the company as a whole.

The Montgomery bus boycott during the US Civil Rights era was extremely successful.