We always enable Mac vs PC multiplayer when it is technically possible however sometimes it is not possible for various reasons. In the case of Shogun it proved impossible, we have two explanations for you a simple one and a more technical one.
Simple answer first
Basically the multiplayer code has some platform dependant features meaning it acts very slightly differently on different OS’s causing sync issues that cannot be resolved without redesigning how the multiplayer code works on all platforms.
More technical answer is some games multiplayer libraries were not designed for cross platform when they were initially created, multiplayer networking is a complex process with many things that need to be kept in exact and perfect sync with other players, this is done by using complex mathematical processes. When a game engine is designed to support cross platform multiplayer care is taken to make sure all of the mathematical processes are done in such a way that they don’t use any platform specific features and the maths libraries and results will always match no matter the platform the engine is built on.
In the case of Shogun 2 the game engine uses the platforms maths libraries to calculate checksums and syncing between players, this means that due to very subtle differences in various platforms maths libraries (floating point calculations specifically) playing games between different platforms will slowly go out of sync due to the small differences ending up with the players dropping out due to sync issues getting too great. Although we did a lot of investigation to see if we could get the Mac platform to act like Windows unfortunately this was not feasible with Shogun 2.
