> None.

This isn't a true statement.

Companies are allowed to argue against unionisation. In the US isn't it even a constitutionally protected right to do so?

There's a huge difference between "argue against" and "actively try to sabotage".

Here (Italy) unionization is a constitutionally protected right. My employer can try to persuade me not to join one, but is absolutely not allowed to retaliate against me if I do, discriminate basing on union membership, etc.

> My employer can try to persuade me not to join one

Yes, and isn't this therefore an example of 'tactics that are ok for a company to combat unions'?