That shouldn't even be a question. You shouldn't need to have working conditions be "so bad" before you consider a union.
Employees are in a vulnerable position. It makes sense for them to join together and jointly hire lawyers and professional negotiators.
A company that forbids you from joining a union is abusive.
I've been in a union and it was super helpful.
Your start-up team wouldn't sign an agreement with a VC without consulting an attorney. Why should your employees sign an employment contract without getting outside advice and help?
This is really good point.
Another anecdote: I’ve been in an unions and it did nothing for us. We were still working minimum wage.