Here are some of the issues I've noticed:
1) Fn+A to open app list, then Tab to scroll thru open apps. The first Tab after Fn+A enables Caps (red light) as if I did Fn+Tab, even though the Tab comes after Fn+A. Then subsequent Tabs do what they should - cycle thru the apps.
2) Once in awhile the device will lock (as in go back to the lock screen, not lock up as in freeze/crash). Unlock with fingerprint, then I'm back where I was.
This happens even though no input - didn't push any key, didn't touch screen. For example, while I was reading a book.
3) A lot of apps don't handle landscape well. It is necessary to Fn+Shift+R. Example: first time I ran Weather Underground.
4) Little plastic pieces on the edge of the hinge start to come off. They're small and finicky. I'm thinking of fixing them in place with a tiny bit of resin epoxy applied with a needle point.
5) Cover Display is a bit of a power drain. However, the thing has a big battery so I've just left CoDi on anyways. It lasts a day even if I do, and I've been using it a lot each day 'cos it's new and I'm playing with it lots.
6) Sometimes an app (or CoDi) starts using lots of power. The front screen warms up. Not sure if this is because the issue is with CoDi or if the main processor happens to be around there. The battery graph shows more power being used (steeper slope down) when this happens. Changing apps (not even closing, just changing focused app) seems to resolve it.
7) I had major problems getting my smartwatch (Samsung Gear S2) to pair and stay connected. Solved with the help of folks in this forum (there's a thread about it). TL;DR is completely disable DuraSpeed and also turn off Battery Optimization for the related apps as well as the Bluetooth app (see thread for details).

Built-in camera app is terrible. Very basic controls, really bad JPEG compression. On the plus side the camera is actually quite good if you use e.g. OpenCamera. Only issue there is that RAW doesn't seem to work but 90%+ JPEG quality gives good results. I came from a Samsung S8+ and that phone (indeed, most devices) apply color-popping filters when they take pics, so if you want your pics to come out like other devices, you'll have to do the same. OpenCamera works really well on the Cosmo though.