Like do they actually, reliably effect change in the way the activists intend?
Have they worked against Israel? Did they work against Apartheid South Africa? Could they work against Trump’s America?
My hunch is that they don’t, really, but can be a useful promotional tool for other issues. Like don’t buy American is a simple message. If people will listen to that, they may listen to reasons why, which maybe could build a movement.
But on the whole I am very sceptical, and would be interested in any reasons for or against boycotts.
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.