I think it is not that simple. The OP seems to have worked with different programming language and paradigms. It also has to do with the type of problem domain you have to deal with. For some type of problems, functional languages are great. For other types, they are simply not.

In my experience things get 'complicated' when you have to deal with a distributed system where multiple actors are querying and changing a shared state. (And that shared state can be stored in one or more databases and/or in files in a file system.)