Why do Banner of Command minions have immunity to magic damage?
If anything, shouldn't it be immunity to physical damage or AoE damage?
I'm guessing it's that Banner of Command minions can survive insta waveclear of champions like Viktor and Anivia, but there are a lot of AD champions with good waveclear (although not as good as those two). Also, they can be killed by minions and towers pretty easily once the other minions are gone.
Banner of Command can be punishing if the enemy team is running heavy AP comp, but heavy AP comp is punishing enough because they can't take objectives easily (with some exceptions). Full AD comps are harder to counter because of their early game strength and strong armor penetration options. Banner could be an effective punishment for such a composition.
What about just heavy damage reduction? Or Bannering the entire wave? Immunity to minion damage? Immunity to magic damage doesn't feel good to use or feel good to counter, and is just wonky in general.
I build AP ONE game...
I don't think people even remember I'm an assassin
Atleast you know what to build.

penetrate more armor than any combination of magic pen items.
and either of its upgrades, as it accounts for total MR as opposed to just bonus, and black clever both takes time to stack up and is build primarily by casters, whom don't even typically get
, the inherent deficiency in MR compared to Armor as mentioned before, as champions get more armor per level, and the large amount of magic based stats available via items compared to AD as mentioned before, and stacking MR becomes a joke compared to stacking armor, especially versus champs that synergize with
.
. Bannor is a full ap comp counter other than that you have 2 MR items tanks can get. Why do you think no body plays adc anymore, its all apc now.