[funcDropShadow]

You are absolutely right! They seam easier to reason about. But relational databases actually are simpler to reason about when you compare equivalent feature sets. I've seen people argue it is just a key value store, so it is dead simple, right? But then they didn't just use key value store features. They used or implemented themselves joins, transactions, consistency constraints under parallel modifications, etc. But this is not the system for which they determined that it is simple.

But even for simple use cases durability might be a requirement. And that is not so simple to get right.

[Zababa]

You're right too, it's hard to know from the start what you will need, and your needs will often grow to the point that you'll need a full relational database.