My experience was very recent. I was working on my game engine, Pillar3D, and realized that the setup allowed it to be automatically multithreaded with little to no concern about deadlock or race conditions. All based on the assumption that individual levels don't talk to each other, and that moving entities between levels could be done between frames. I can even track about how much work each thread has to do and use that to distribute levels among the threads. Now I can do things like force UI trees to exist in their own level and get fantastic multithreading.