The test of whether code is sound: can multiple people successfully own the system?

That is, maintain, fix, and extend it with an accurate mental model?

If not, it's not known to be sound.

Soundness normally requires careful layering, making sure that each layer is thin and understandable.

Thinner layers are also easier to communicate and explain to people; instead of having to communicate a multi-ply idea, you can communicate a succession of single-ply ideas.