I asked this same question previously and is something I really want to understand as well.
I use the Pimax 4K with no controllers and AUTOPLAYER VR is by far the best despite it being absolutely no frills - sometimes less is more.
I mainly watch VR SBS 4K/5K videos which requires 360/180 tracking and AutoplayerVR does the job perfect. There are only a few issues in that it doesnt have a zoom function so I can get the video to match real life - I think the reason why this is necessary speaks for itself.
I've used the more full featured players but find they all have issues in one way or another, eg. not playing certain video formats with Win8.1, requiring hands/controllers (again speaks for itself), multi-screen to switch videos, etc etc.
I really hope PIMAX will have AutoplayerVR support the 8K with its dual LED Panel and 180/360 deg. tracking as the software works well with the Plmax 4K but I am not sure if it will support the videos you download off the net which is designed for VIVE and Oculus Rift in mind, which is only a single panel screen.