I will let Contrivance speak more about balance when he gets back, but hard limits and resource costs will prevent players from spamming Jagdtigers and such.
Time to speak about balance!
Balance is a tricky topic since our approach is quite different from the traditional RTS Gather Resource -> Upgrade Tech -> Build Unit framework that CoH has. We took a more tabletop wargame approach to the gameplay, meaning that we eliminate the majority of the economy and focused more on an attrition model.
What this means is that you won't be capturing territory sectors for resources, but instead capturing them as part of your victory objectives. Your Manpower resource will be the only resource needed to purchase units, but there will be a cap on how much you can earn, and it will steadily trickle in until the cap is hit. Also all your units will have their own hard limits, meaning that you cannot endlessly replace your losses of a specific unit even if you have the Manpower to do so.
Balancing infantry vs tanks is also important, since some Battlegroups will be more infantry-focused and will need to still be viable when facing against massed tank attacks. Fortunately history has already shown that infantry forces can face and defeat tank assaults, so we've been tweaking things towards that. Even Red Army troops armed only with RPG-40 or RPG-43 anti-tank grenades will be a credible threat to a Tiger if it blunders through an ambush without any escort.
As for balancing German ubertech against the Allies' equipment, the unit costs paired up with a more realistic approach to combat should do the trick there. While a Tiger II will be incredibly hard to knock out from the front (though spalling from high caliber HE shells could cause some damage), shots to the side and rear from most medium caliber guns have a much better chance. A single penetrating hit could cripple or even destroy it, and since the Tiger II will be a costly investment, an American or Soviet player can afford to lose a unit or two in the process and still come out ahead. Someone spamming Tiger IIs without any combined arms support is basically begging to lose.
Choosing a Battlegroup is the first thing you have to do, and this choice determines what units you will have available to pick from. Everything not available to the Battlegroup is locked and hidden away in the menus, to keep the clutter to a minimum. Additionally you might have choices to make in regards to certain units, such as what model of Panzer is in that Panzer platoon you want to deploy, or what sort of artillery piece is in the battery you want support from. Choices like these will lock out the other possibilities, so choosing the PzKpfW "Panther" Ausf.G means you won't be able to field the much cheaper PzKpfW IV Ausf.J later. The information on how locks out what, and the unit limits, will be shown and updated in the button itself so that there's no unpleasant surprises.
As for an example of a Battlegroup, one that I'm working on at the moment is the Razvedki Battlegroup. This is styled after the Soviet Reconnaissance units that raced ahead of the main force to gather intelligence and capture objectives by speed and surprise. The main core of it is the infantry of a Razvedki company, which are unique in that they had armoured vehicles to transport their 9-man squad. Since Soviet vehicle production couldn't spare the resources to build armoured transports, Razvedki squads ride in lend-lease M3A1 Scout Cars, Universal Carriers, or captured Sd Kfz 251/1 half-tracks (Motorcycle squads were also available, but unfortunately we don't have a motorcycle with sidecar model). The rest of the Battlegroup consists of fast-moving elements that could keep up with a reconnaissance force, so you'll have tanks, assault guns, motorized and towed guns, and artillery support. Since the core unit is the Razvedki, you will have a limit of 8 squads (one company) rather than just 4 squads (one platoon) which other Battlegroups will be limited to.
The theme of the force is bold and daring attacks mixed with scouting, since Razvedki units did both in equal measure. The Razvedki squads themselves will have both scouting capabilities - allowing them to spot enemies in camouflage as well as make them appear on the minimap outside of your normal view range - and will be very capable soldiers on top of that.