More insights from John on Barbarossa to Berlin and how well it models the real thing:

My Normandy example is possibly not the best. The game limits you to turn
14 (Spring 1944) or later. This in itself is somewhat arbitrary but not
unreasonable. The key point is probably that you should not be able to play
Overlord until a certain number of turns after ASW Victory although this
probably applies to all of the invasions outside the Med. For Torch they all
had to scuttle across the mid-Atlantic off the trade routes to concentrate
at Gibraltar and hope that the U-boats didn't notice. A concentration of
transport shipping was obviously just what the U-boats were looking for and
at best surprise would have been totally lost.

With regard to the turn limit on Overlord, if you look at the COSSAC plan
for the invasion of France it is clear that until Montgomery arrives from
Italy what is being considered is a much smaller scale affair with only 3
divisions landing in the first wave rather than five. Because of this and
the option of doing Round-Up instead I can live with the turn limit on
Overlord.

Other things do not come through in the game however. One of the major
reasons for invading Italy was the opportunity to route shipping through the
Mediterranean and Suez rather than round the Cape. Since the single
critical limitation on Allied operations in all theatres was the available
merchant shipping to deploy and supply forces, saving ton-miles by invading
Italy significantly increased the speed with which the build-up for the
invasion of Europe could be conducted.

Other oddities are the frequency of Stalin Orders - fine in 1941-42 but
totally wrong later on (unless there is a rule or card I have missed).
Stalin was much more ready than Hitler to listen to his generals once they
had proved themselves (he just consoled himself with the thought that after
the war he could have them all liquidated). Likewise it is a little unkind
to only allow the Soviets Mechanised Fronts after Lend-Lease. The trucks
are important in allowing the Soviets to carry on operations in operational
depth - their previous offensives had a tendency to peter out in the first
couple of weeks after the breakthrough. This partly disappears by the time
of the Ukrainian offensives of late 1943 and entirely disappears with
Operation Bagration in 1944 but is not just due to US trucks. A significant
factor is also internal organisation of the Tank Corps and the problems of
low availability of armour due to breakdowns and poorly organised support
services. So Lend-Lease as the route to Mechanised Fronts does not entirely
do the Russian war machine justice.