For brigades:
Please keep in mind that TRP combat is different from DH. We have reworked and redone the combat values and combat math. So any brigade has some spot and advantages/disadvantages. Some of them performed better in MP and some performe better in SP.
E.g. in MP games we have seen that a british player will invest into heavy tank brigades for armored division because they are the best to save manpower and they will add the best modifiers to stay in battle against axis armored divisions.
For the air units; Well. There are a number of elements.
First of all we have worked with air wings around 100 aircraft per division. So an air general can command 500 aircraft in combat (5 wings). That is already pretty impressive. The Marshall can command 8 air wings without commanding penalty. For more wings u have to look into advantages/disadvantages of overstacking.
Furthermore is a balance element to balance airpower for MP games (strategical bombing, air raids, airfleet wipes, ectpp).
For the speed; Why should we add a speed limit? Those tanks should be able to move as fast as the divisions. There is no reason why they should slow down a division.