For your video playback, if 3rd party apps cannot resolve them, then it will be the same for any other similar level specs NAS brand, unless you go for the higher-end processors which is costly.
Asustor advertises the NAS as if you can throw anything at it and play back any video format on any device because the NAS will transcode on the fly if needed. I purchased the new AS 5402T to have an even stronger processor to be on the safe side. But my reality is that I keep getting error messages on the server or on the client side or have to wait ages to play back mere FHD videos on the NAS itself or on a relatively modern phone (S21), not to mention a TV. So I am not talking about heavy lifting of 4K h.265 videos (that are advertised as well), but plain old 1080p/h.264. I don't want to convert all my private videos because I don't want to touch them (including the file creation date), and I also don't want to waste storage space on converted copies that I would also have to keep in sync with the originals. 
As for your hibernation and sleep issues, it should be one of the apps you're running or ADM can't process one or more of your media. Mdadm will always be running even if you're not using raid. I'm using both ext4/ btrfs and I don't have these issue, probably due to my mods. You can search the forum on how to troubleshoot.
I think it would be Asustor's job to implement sleep mode in a way that it overrides all processes so that the user doen't have to care. If a multimedia app or a system process regularly scans the drives, there should at least be a user friendly option in the OS to suppress that to enable sleep. I don't have the time and am not willing to become an ADM/Linux professional to search for processes and to tweak the system only to make it perform an advertised function. This should work out of the box! I have already spent way too much time troubleshooting this brand new device. 
The problem is: I don't see a real alternative in terms of hardware, in particular looking at the four M.2 NVMe slots. I don't like Synology and their outdated hardware, but could I expect that these things work on Synology? Or QNAP? Or am I struggling with the concept of a NAS generally?
 
			
			
									
						
							AS5402T, 4 GB RAM, Volume 1: SSD Lexar NM790 4 TB, Volume 2: HDD Seagate Exos X18 18 TB