Concerns about using ZeroBounce range from permission practices and historical aggressive tactics to inaccuracies, limited integration options, and data privacy. Specifically, reports indicate unauthorized addition of emails without consent, past use of unethical techniques like dictionary attacks, potential for false positives and questionable accuracy in validation, slow and unhelpful customer support, high costs particularly for large lists, data privacy concerns related to GDPR compliance, limited ability to detect spam traps, less extensive integration with platforms compared to competitors, and the fact that using ZeroBounce doesn't automatically guarantee sender reputation improvement. Experts also highlight potential abuse of SMTP protocols and the importance of verifying results with other tools.