Employers won't hold it against you for being against tactics that involve privacy and surveillance issues in general (eg: "I wouldn't want to work at a place that spies on every single Slack message I send to coworkers"), because they give you the benefit of doubt that from their bottom line perspective you will be a net-positive contributor. The minute you include the word U-N-I-O... (eg: I wouldn't want to work at a place that spies on unionization efforts), that benefit of doubt is lost, and now everyone is wondering why that topic is on your mind.

None of this is a reflection of the value of unions from the employees' perspective. But it's important to realize that unions are one of the instances where the employees' and employers' interests are not aligned. So, emailing a recruiter and mentioning unions has only one possible outcome - none.