Hi @Charles_Fraser,
I wrote down some notes based on some preliminary testing I did with multiplayer support with Cesium for Unreal here: Multiplayer Origin Rebasing Support Notes · Issue #480 · CesiumGS/cesium-unreal · GitHub.
The issues and limitations you may run into will depend on the use case - how big of a playable area you are designing, what kind of servers, and whether anti-cheating is needed (players being able to get to arbitrary locations by changing their gravity, movement speed, etc.)
The playable area will inform whether you want to use multiplayer origin rebasing or not. For a play area smaller than 2km or so, it is probably not necessary, from 2-20km it would be recommended, for 20+ km it will be difficult to keep players in a stable, accurate position relative to the globe regardless due to limitations in Unreal’s networking, without modifying the source.
If the player characters will have gravity, it will be important to set bIgnoreClientMovementErrorChecksAndCorrection=true and bServerAcceptClientAuthoritativePosition=true. I describe this in a comment in that linked github issue as well.
Happy to answer in more specifics as you develop. Best of luck!