I dunno. Even in embedded systems every time I've started without a database I've eventually come to need something like a database, and in every case I've found myself building essentially an ad-hoc poorly managed database into the application including marshalling/unmarshalling, file management, notification, and so on because each new feature over the top of regular files was just that much easier to add versus switching to a database system.

However the driving motivation for adding a database is not necessarily managing data, but the fact that the database system creates a nice abstraction layer around storing data of relational or non-relational form in non-volatile memory and controlling access to it while other systems are updating it. And because it's a nice abstraction, there are a lot of existing libraries that can take advantage of it in your language of choice without requiring you to completely invent all of that stuff over the top of the filesystem. That has knock-on effects when you're trying to add new functionality or new interaction patterns to an existing system.

And in cases where two or more processes need to communicate using the same data, a database gives you some good abstractions and synchronization primitives that make sense, whereas regular files or IPC require you to invent a lot of that stuff. You could use messaging to communicate updates to data but now you have two copies of everything, and you have to somehow atomize the updates so that either copy is consistent for a point in time. Why not use a database?

Knowing what I know today I would start with some kind of database abstraction even if it's not necessarily designed for transactional data, and I would make sure it handled the numerous concerns I have around data sharing, consistency, atomicity, and notification because if I don't have those things I eventually have to invent them to solve the reliability problems I otherwise run in to without them.