The debate on unionisation in prostitution is deeply rooted in whether or not prostitution is considered a legal profession.
In many nations, it is not.
Unions would not make sex work safer for any party involved. The legalisation of prostitution and unions would only allow for further exploitation under the cover of the law. 
Unions would make abuse and violence internally amongst sex workers harder to prosecute.
In the prostitution industry, workers operate as independent contractors, often without bosses or supervisors. Unions would not be possible or effective within the current system. It is not feasible to unionise an industry that is extremely difficult to track or regulate.