I am not going to say all unions are bad. If done properly a union should be a huge benefit to workers. Unfortunately, in the US many of them are basically scams designed to enrich the union officers as much as possible.
My brother works for a union company and is a member of the union. He has observed over the years that many of the union's actions are much better for the union and the union officers than they are for the union membership. The problem in his view isn't the idea of the union, so much as the fact that many of the union members really don't understand these contracts being negotiated on their behalf. What most of the members know of the union's actions is from the union newspaper which only ever reports glowing successes. So you have an employee base most of whom don't understand the deals negotiated on their behalf and whose only information to judge their leadership is from a newspaper written by that leadership. So when election time comes around they invariably pick the guy that the leadership nominated for the job. Other people run but the pretty much never win. As long as that is the case US unions will continue to be extremely corrupt.
As far as Tesla and Mr. Musk go if they don't want to be unionized they only have two real options. One is to pay enough that the union can't really promise their employees enough to make them vote for a union. The other option is to move to a right to work state. As far as the union sending in a guy to stir up trouble he is going to have to understand that is how they operate and that will continue. He's just going to have to accept that and take measures to make people not want to vote for a union.