We’ve been using Serverless with Sequelize 3 for years in Lambda. It works fine. There’s a connection pool issue in Sequelize 4 that was just fixed recently to make it work for Lambda as well. I don’t know if it’s integrated yet.
We’re not using Webpack on the back end (just our front end) so maybe that’s why we never had an issue with the MySQL 2 library. We certainly didn’t have to recompile anything and most of our developers are on Windows.