I do test manually in salesforce. Mainly its because you do not control everything and I find the best test is to log in as the user and go through the screens as they do. I built up some selenium scripts to do testing.

In old days, for the kinds of things I had to work on, I would test manually. Usually it is a piece of code that acts as glue to transform multiple data sources in different formats into a database to be used by another piece of code.

Or a aws lambda that had to ingest a json and make a determination about what to do, send an email, change a flag, that sort of thing.

Not saying mock testing is bad. Just seems like overkill for the kinds of things I worked on.