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

Generally, I think it is a bad idea. Unit tests guide the developer to write modular (and therefore useful, reusable) code because their code needs to work with both the production system and the test code.