This is a small list, was just getting started when the game kept crashing too much for us, and we finally stopped playing for a bit.
1. Secondary player joining games hot bar does not save in memory or saved saves, unless secondary is kicked out and comes back in while host is still active.
2. If a secondary player joins after a few milestones have been achieved, they do not get access to any of the milestones the host has already achieved. Host player had double the inventory space, secondary did not.
3. Building 8x2 floors sometimes do not show up as built for secondary player. Host can see the floor, and secondary player can walk across it, stutter stepping on it, while it looks like a hole still to secondary player. Logging out of game and coming back in fixes this.
4. Secondary player lagged while driving a truck, game crashed, upon re-entry into game, was inside truck at bottom of a large killing drop, still alive and in truck. When he looked around, he was able to get a bottom up view through the floor to the underside of truck, looked like a glass ceiling under the truck itself.