Regarding the detailed mathematics:
The "200% or x2" model for oneself = "50% or /2" for the opponent works when there are only 2 units, one of ours and one of the enemy, and no other active participants (including heroes).
As soon as other participants (units and heroes) are added, the model starts to work differently, based on the sum. For example:
- increasing the initiative of 1 unit by 2 times = increasing its strength by 1.5 times (+50% of the unit)
- decreasing the initiative of 1 unit by 2 times = decreasing its strength by 1.5 times (-33% of the unit)
If, for example, we have approximately 4 equal units on each side, we get a change in the balance of power in the first case from 4:4 to 4.5:4 (112.5%), and in the second case to 4:3.67 (109%). Obviously, in the first case, the relative size of the army increases more.
Furthermore, duration also matters. A simple example: if we accelerate a unit with initiative 10 by 2 times, it will perform 2 actions in 1 round (+1 extra action). And if we slow down a unit with initiative 10 by 2 times, it will only lose its full action after 2 rounds (performing 1 action instead of 2).
In short, "similar" buffs work more powerfully and have a faster impact, while "similar" debuffs work less effectively and have a slower impact. This is the fundamental difference, which is very noticeable in practice with large numbers (on the order of doubling the initiative).