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

Having a person dedicated to unit-testing in a team can seem interesting at first sight ; as he will review all the code. This also would alleviate the risk of having one developer making the same error in the production code and in the unit tests.

However, in order to be unit-tested, the code has to be testable ; what can he done when the code it receives is untestable ? Rewrite it ? Refactor it ? Without Unit Tests ?

In short, I think this would not be such a good idea.