From this thread:https://forums.totalwar.com/discussion/268139/auras-skills-balance-overwiev#latest
Charge bonus buffs are just hard to time and position, and are limited:
You have to use it in a small time frame (before the charge)
often the bonus is too little for its cost
the size of the aura is small that could only buff itself and 1 other unit reliably.
It buffs well on specific units only unlike WS and MA/MD buffs that is always helpful regardless for chaffs/elites.
Most of the battle it doesnt come useful. CB effect only last ten seconds that degrades too so that sweet 80 CB you see is only in the first second in combat.
Look at Thane, and Alberic's CB buff, never taken
We already have some CB Buff map wide buff like Lore of Wild passive, Orion's horn and they arent broken.
Though they are activated and others are permanent aura, regardless, they are too limited and unhelpful
the numbers can always be tweaked if needed but i doubt it since their numbers are often so little anyways.