The group “Recently modified” is synced across all devices, as one expects, since modification time is a unique property of a file.
On the other hand, as far as I can tell, the group “Recently viewed” lists recently opened files as they were opened on a specific device/DT install. This is not unreasonable, opening time is not entirely unique to the file.
However, I would find it very useful to have “recently viewed” also synced across all my devices, as I frequently hand off files from one to the other. E.g. I edit a file on the computer at home, read/annotate it on an iPad, check it on the iPhone while commuting, and then maybe print it from the computer at work. “Recently viewed” matches this workflow perfectly, if it can show the time-ordered list of viewing instances across all synced devices. I feel that this is in the spirit of a modern, cloud-based, workflow, where we can focus on the content, not the device.
Right now, I use flagging and color labelling to keep “hot” files at my fingertips, but this would be much easier.