It probably help authors of those modules, or at least "emailer-local", if you could find what's different in settings in database. Maybe it's just a tiny option, that could be added to emailer-local and it would work in both cases?
Looking into both modules package.json and index.js it looks like https://github.com/NodeBB/nodebb-plugin-emailer-local/ does not install "nodemailer-smtp-transport" like https://github.com/erlgo/nodebb-plugin-emailer-ssl-smtp does by default. Maybe it was just a case of missing dependency?
As for sending mail, emailer-ssl-smtp adds
secure: true when setting up transport, but that does not seem to be stored anyway, so shouldn't be used by emailer-local.
Also transport initialization is done a bit differently, but that too shouldn't matter because it's not "shared" in any way between modules.