I don't trust the anonymity and without that you can't trust the honesty. These surveys always get sent out with some sort of id/token in an email and after that you're feeding data into a black box where you have no idea what kind of information is being stored and how much of that information is available to the creators the survey.

If there were some sort of neutral third party (like a non-profit or maybe a union) running the survey I might have some trust, but companies in this space like survey monkey are not neutral, they're beholden to whoever pays them.