Unit tests, while good at promoting decoupling, can absolutely be a major driver of complexity, as it may break the code into far more units than what is reasonable.

Be careful with this. Unit tests don't tell you much about the correctness of a system overall, and they rarely survive a substantial refactoring. Optimizing for unit testability can make individual classes/functions "simple" but at the expense of creating a ton of them and pushing the complexity to the interfaces and integration between them.

I love unit tests, but admit I have absolutely seen unnecessary complexity including complete classes and namespaces solely to enable testability in many cases.

It's a justifiable trade off for me, but I don't pretend that unit testing reduces complexity.

My experience with automated testing was great until I had to test I/O functionality: files, databases. That's when the test suite itself became too complicated.

Absolutely! For me, comprehensive testing is key to keep things clean over time. Not sure why this didn't come to my mind when writing the article. I think I was somehow assuming that this is a necessary pre-condition anyway.