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

I think it's generally bad for QA to write unit tests. Unit tests are code, they are tightly related to how the dev code is constructed. For this reason, the developer knows best what tests would make the most sense.

On the other hand, I believe QA should stay as close as possible to unit testing. QA needs a good relationship with the dev and try to understand the code design. Why? When a developer is writing unit tests, the focus is still on development, not testing. You cannot trust (not in the bad sense) a developer to come up with the best test cases and that she has good coverage. Since QA is specialized in testing, it may be a good idea to keep QA and dev as close as possible during unit testing.