I am not much of a devops person but running your own DB in a VPS with docker containers don't you also need to handle all this manually too?

1) Creating and restoring backups

2) Unoptimized disk access for db usage (can't be done from docker?)

3) Disk failure due to non-standard use-case

4) Sharding is quite difficult to set up

5) Monitoring is quite different from normal server monitoring

But surely, for a small app that can run one big server for the DB is probably still much cheaper. I just wonder how hard it really is and how often you actually run into problems.

My guess is some people have never worked with the constraints of time and reliability. They think setting up a database is just running a few commands from a tutorial, or they're very experienced and understand the pitfalls well; most people don't fall into the latter category.

But to answer your question: running your own DB is hard if you don't want to lose or corrupt your data. AWS is reliable and relatively cheap, at least during the bootstrapping and scaling stages.