On your screenshots the roughness looks pretty similar, between painter, Unity and Unreal (if you look at the specular highlights on the edges, it's identical).
Most of the difference I see here is likely more due to a difference in tonmapping, whcih crushes the blacks in Unity and Unreal. If I'm not mistaken, both use AcesCG for tonemapping while Painter has no tonemap by default and is more neutral.
We will likely ship Painter with similar tonemapping options in the future to match the engines more closely.