You must have some special knowledge to fill in right values in right parameter boxes. Therefore, Autodesk was seeking for a solution, that would make life easier for simple users. They came up with a technique using physically-based rendering and it is implemented in AutoCAD 2016. It simulates reality using principles of physics and therefore requires less settings for good results. That is true – it is more simple to use and it gives good results!
But if you want better results, similar to what Mental ray was capable, then you maybe will not be 100% satisfied with new render engine. Mental ray did require more knowledge, but it also offered more choice.
If you want it back in AutoCAD 2016, simply type in the variable RENDERENGINE and set it to 0. When the RENDERENGINE system variable is set to 0, the mental ray rendering engine and workflows are enabled.
That way you can go on rendering as you were used to in previous releases.