I've not had the selective rendering problem, but I do have scenes that P8 just won't render but P6 will.
In the most recent case I got an error message, but I have several where the render window appears, t starts to list the process then disappears and leaves a black screen in the render window.
I'm starting to think P8 is growing its own bugs as I use it. The latest weird one, if I launch a pz3 by name from My Recent Doc's in the Start menu it loads with a Language Bar at the top of the screen, works for a few minutes then crashes to desktop.