This is something I also struggle with a dozen+ times a day (6 core mac pro trashcan), it doesn't matter what project or the complexity of it. I would say 95% of the time if I render a viewport in renderworks, then change the lighting setting on the viewport (IE change brightness from 35%-50%), the result will be it won't render and a couple seconds after pressing render it will just show a black viewport, and nothing can fix it unless you restart the app. It is very annoying if I'm trying to render out multiple viewports for publishing and I've already rendered a few before this happens, as I then have to re-render the other viewports as well after restarting. This happens in 2018 but used to happen to me in 2017 as well before I updated.
I have also had the issue that chris0325 describes, though less frequently than the other issue, I make changes to my drawing but rendering with renderworks shows the state of the drawing from earlier before changes were made.
Maybe a temporary fix for the Vectorworks team until they can sort this out fully would be to give us a menu option to restart the Cinema4D renderer in the background without requiring us to shutdown Vectorworks. I've tried force quitting it in activity monitor but it doesn't restart up, then Vectorworks doesn't even attempt to do any rendering after it has been force quit.