There are some minor deployment issues with the stock 1915, 1916 and 1918 scenarios, some allied units do not appear on the map although they are incorporated into the game files. Cannot comment on the other scenarios outside of the Grand Campaigns. These come in two flavours: 4-Player (Germany or Austria-Hungary or France/Britain or Russia) and 2-Player (Central Powers and allies or Entente and allies). The four-Player campaign is about as close to playing a single country as you can get but as I recall, you are required to also play certain minor nations like Turkey or Serbia. Don't take that to the bank however, it's been quite a while since I tried a 4-person campaign.
If you're comfortable I would say go for the big prize; a full Grand Campaign that can run up to 50-turns, ending with the November-December 1919 turn.
Some threads that discuss modding the stock campaign may be found here: (Insert shameless plug...)
Modding with JSGME
JMass Graphics Mod
Increases map clarity.
Battleships to Turkey Event Fix
This was supposed to have made it into the final 1.08Q patch but that may not have happened. The fixed files are in Post #6.
Missing Brest Fortress Fix
Fixes a couple of missing fortresses in Russia.
Be advised that many Players seem happy with Stock so do not automatically jump into WW1G
mods but I would recommend the Fortress and Battleships to Turkey fixes and JMass' excellent graphics enhancement.
The first couple of turns can be complex and the variety of war plans and war plan options really help re-playability but also means that the war that you fight may or may not repeat the Schleiffen Plan, Plan XVII, the Russian invasion of East Prussia and Conrad's Great Gaffe.