It's a good idea, but the proper distribution of bonuses is a problem.
Why the German armor is better? Their technique is good, but the best quality metal was in Britain, their armor had the best properties. A Russian armored vehicles had to be penetrated worse, but it was very nice to drive what makes them effective. All these countries provide a bonus?
Why Britain bonus ships? Remember the battle of Bismarck and Hood and other battles on the water, where it is obvious that no advantage was not. Often, they have won at the expense of the numerical superiority. If you recall the submarines, it is obvious that the sea has to be a bonus from the Germans. But they had great difficulty in the construction of large ships. Maybe the UK should be a bonus in terms of construction of large ships? But America quickly built small ships, and Canada too quickly build commercial ships, Japan quickly build aircraft carriers. It makes no sense to give a bonus to some one.
What to do with minor countries? What do you give a bonus of Yugoslavia or Algeria?
Maybe let the player at the start of the game to choose a bonus?
In addition, if we have something to raise the bonus, we have to do something to reduce by a penalty.