Hi there rfollett,
Thanks for posting. This plugin here (MailChimp List Subscribe Form plugin) is the only WordPress plugin for MailChimp that we MailChimp tech support chimps are able to directly troubleshoot. I can’t really speak specifically for the differences between our plugin and other plugins, or assist with troubleshooting those other plugins.
Generally speaking, some plugins us an API key from your MailChimp account, whereas some use oAuth where you log in using your MailChimp credentials, and there will likely be some differences between how you can display list fields and add subscribers to the list.
Let me know if you need any assistance with the MailChimp List Subscribe Form plugin here and I’ll be happy to help out. Have a great day!
-Kath