The game has countless replayability options.
A single game takes a lot of time to develop and every one of them follows a different path, specially if you want to pick a different nation at a start of new game.
There's a lot of complexities to deal with regarding managing your nation and getting along with others as well.~
Requires time, patience and learning, since there are many options to deal with.

The speedup of the game gets progressively slower as time goes on, as it appears is cpu dependant, which also makes it slower to progress in the game.

As for unlocking achievements, you gotta select playing on iron mode on at least the regular difficulty. So they are hard to unlock as well.