The one that gets me a lot, which is similar in practice to your point, is when I need server redundancy, even if one server is otherwise plenty for my task. As soon as I'm not running in one place, you need network data storage, and that kicks pretty hard in the direction of a network-accessible database. S3 works sometimes and the recent work on being able to atomically claim files has helped with some of the worst rough edges but it still doesn't take a lot to disqualify it, at least as the only store.