"databases are harder to understand than simple nested data structures"

I would debate that (subject to the availability of cold beverages).

In most cases I have encountered, relational databases offer a simpler, more directly-mapped representation of the problem space, when compared to multi-level data structures, especially when it comes to traversing said structures to get summaries. I find OOP is required to approach equivalent simplicity.

I'm too lazy to substantiate that argument, so I'll just leave it there, and let you have the last word, should you so desire.

Cheers!