Overall the AI is fair from the tactical point of view .
The worst thing it does tactically is to counter and push as soon as there is an open space in the front .
A human player will destroy easily number of soviet units by simply opening a window in his front and then closing it when enough soviet units went enthousiastically into the (future) pocket .
However the purpose isn't here the tactics but the very basics of strategy where the AI is desperately bad .
I will use an example of a campaign won by Germany using the (France->UK->USSR) strategy .
If there was not US, this strategy would always ncessarily win .
So the one and only key for Allied to prevent a German win relies on the USA .
Follows (again necessarily) that the US strategy is a translatlantic Invasion .
The location is not so important (UK, Ireland, France, N.Africa) what is important is that an invasion happens .
And the necessary and sufficient condition to make a transatlantic invasion is to have naval superiority .
So what is the strategy to achieve a naval superiority against Germany ?
The naval combat in SC is a simple rock-paper-scissors game .
Subs beat major ships . Destroyers beat subs . Major ships beat destroyers .
Germany being strong in subs, its strategy to naval domination is to destroy ennemy destroyers .
Follows that the US/UK strategy must be to build destroyers and to destroy German subs .
The one who achieves his strategical target first wins the war .
In my game Germany successfuly destroyed first the UK destroyers (what enabled the invasion of UK) and the US destroyers second what enabled the Atlantic domination .
What did the AI in the meantime ?
First is the screen shot of the US production queue when the war was finished .

Everything is totally useless . Not a single destroyer being produced . The same is true for the remnant of UK in Canada .
So what is the AI doing ? Second screen shot .

Well it is sending units across the Atlantic . As one can see, they get all slaughtered by the German waiting subs .
So apparently the AI is just blindly sending units across the Atlantic , not realizing that none survive the Journey .
And what did the US do since their entrance in the war in 41 ? Third screen shot .

Well they have just been producing and storing tons and tons of planes , tanks and infantry in the US . Note that the UK in Canada has been doing the same thing .
This is not only useless but it is incredibly dumb . Such a strategy is no threat to Germany and it is not even a correct defensive strategy .
If the game continued after the fall of USSR, Germany would simply invade North America . It would be only a matter of time untill the experienced and elite units just punch through this inert mass of UK/US units .
Conclusion and suggestion .
To significantly improve the AI performance on the naval strategy field, it would be enough to make it realize that the naval combat is a paper-rock-scissors game and to act accordingly .
Basically in the WWII setting that means that what counts are ONLY the "Allied destroyers/German subs" and the "Allied destroyers/German major ships" ratios .
The closer they get to 0 , the more certain is the German Victory .
That's why the only Allied strategy enabling them to survive and perhaps even to win is to research, produce and manoeuver in such a way that this ratio stays around 1 and if possible above 1 .
If it falls below 0.5, then the war is strategically over .