I was dealing with pretty complicated motion comps with lots of 3D layers, so I never tried to play them without rendering. If I zoomed out/in, I had to sit an wait for about 30 seconds for all the video and audio layers to refresh thumbnails/waveforms, like they weren’t being held in cache. Rainbow balls appeared throughout, and sometimes resulted in crashes.
Perhaps some of this instability will be addressed in 10.5.2.