Networking
Unreal has a very good networking system. However, it is not designed to work at large scale unit counts.
While player-specific data (resources, spell points, team...) can be replicated. Taking inspiration in Lockstep systems (read this article about network code in Age of Empires), we are testing and implementing a non-deterministic, client-server system based on commands and low periodic state updates, which allows us to keep bandwidth low.