I've personally migrated projects from MySQL to PostgreSQL and I'm currently working with teams that are moving wholesale from MS SQL Server to MySQL.
I tend to abhor full ORMs that require layering deep knowledge of the ORM's idioms on top of my existing knowledge of SQL and the underlying SQL implementation. An ORM doesn't magically free me from understanding the actual implementation and the additional overhead often isn't worth it.
I tend to abhor full ORMs that require layering deep knowledge of the ORM's idioms on top of my existing knowledge of SQL and the underlying SQL implementation. An ORM doesn't magically free me from understanding the actual implementation and the additional overhead often isn't worth it.