Unit tests by a QA Engineer

Question

As I understood from some Q&A sessions (see this and this), unit tests should be written by developers.

At my previous workplace we tried to give this task to a QA Engine

Answer 1

Where I work we all do unit testing but not on your own code. You still write testable code and will still focus on quality. Developers become more familiar with areas of the software they are not working on. Everybody knows the code base and can take over from other developers.

For a non developer of the software to write unit testing I agree with all the other answers. It is a bad idea.

In your question you said that you had one person doing the unit testing. Good unit testing requires a lot of effort depending on the test coverage required. It is anywhere from 50% to 100% of the level of effort of the development team. One person testing a lot of developers code will be totally overwhelmed.