unwanted strikes - if your union goes on strike, you are not allowed to work
special treatment - unions try to equalize, so higher performers may not be fairly compensated
An awesome employer shouldn’t discourage unionization, and ideally they’d encourage attempts to unionize, but they wouldn’t recommend unionization, assuming the employer intended to maintain control and monitor managers throughout the chain. If the employer can provide all of the benefits employees would get through unionization, unionizing merely adds extra BS that employees and employers need to deal with.