You need the available hosts to be within the same class of opportunity cost in order to decide what aura this unit should give.
If UC is an available host, then command vehicle should get a completely ineffective aura, like the one that Vipper suggested. That's because the cost of UC + Command Aura is too cheap for anything bigger.
Alternatively, yeah. I think it would be OK if the strength of the aura scaled with the strength of the vehicle committed to it.
However, creating 3 different auras for 3 different vehicle classes (UC, AEC and everything else), requires us to get 3 things right at once. However, when we're juggling with 10 commanders at the same time, complexity can be a dangerous enemy.
So, we'll try to optimize the aura for a use-case where people can use it on any medium+ tanks, probably. That's because there's a large variety of expensive tanks to choose from, and that's what CoH1 had after all.
So what if it's cheap to convert a UC into a command vehicle? It is incredibly easy to kill if they use it aggressively to take advantage of the aura. That's the risk/reward.
If I want a command vehicle that can stay in the front lines I'll convert a tank into one.