I question a lot of advice like this, and agree with you. I’d generally far rather use NSubstitute than have to write fake classes, but I can see the utility in some cases.

My honest response is this: Writing tests should be quick and painless. Maintaining fakes is more painful than maintaining substitutes. Substitutes let me interrogate side effects in great detail. I’m not concerned will performance in unit tests. Ergo, substitutes are quicker and less painful, so IMO that’s a win.

But for real though, why should anyone care about someone else’s test architecture, of all things. As long as you neither under- or over-testing, why on earth does it matter?