It does matter. Each exception is increasing complexity and onboarding cost for new users. You need to seriously look at whether an exception carries justifiable merits for the game’s health to warrant being included. Such an exception for mercy does not. It’s just there to enable lazy game design/balance.
It’s not “ah ok mercy just randomly does 33% the damage against ACUs” it’s “ah ok, time to check if every other unit in the game lies to me”
Like revert beetle to what it was and make it do 25% damage to ACUs.
Make notha do 40% damage to ACUs and buff the aoe so it’s more intuitive to snipe with it. But those damage differences aren’t how things like notha get balanced because it’s unintuitive to randomly introduce armor like that unless it becomes a core game mechanic. It’s only currently in the game to prevent totally toxic unintended interactions against asf or ACU as you mentioned for example. You don’t intuitively think to groundfire czar against asf but you do intuitively think to use the aeon magic snipe missile against the autowin game target.