1) Am I testing wrong if I am trying to mock data classes in kotlin? 2) If the answer for first question is no, I am having issues mocking it with Mockito 2 even after following this article by Haidi hariri. http://hadihariri.com/2016/10/04/Mocking-Kotlin-With-Mockito/ Do I have to do something on top of it?

2. Just as remark about your problem. You can mock final classes using Mockito in unit test, it’s impossible for Android Instrumentation tests.

or just apply the kotlin "open-all" plugin ?

This is also solution, but you should understand that now is no way to apply open-all only for tests, so all your classes annotated with open-all annotation will be opened.

what is pros and cons if I do that ?

Those classes can be extended by any client of your library for example or inside your code using reflections