Not sure how you can really say that. Mispredictions are mispredictions, and their being dangerous isn't just a factor of it being that it's vs. Aegis in the least. Predicting Garchomp to go EQ when it ends up going Outrage is a huge problem if you switch to something that doesn't resist dragon and it ends up going for Outrage. Which, incidentally, is stronger than Aegislash's Shadow Ball:This is the 50/50 situation other users warned about - other Pokemon can set this up too, but with no other Pokemon is the consequence of predicting wrong so dangerous.
252 Atk Garchomp Outrage vs. 0 HP / 0 Def Shaymin: 196-232 (57.4 - 68%) -- guaranteed 2HKO
252+ SpA Aegislash-Blade Shadow Ball vs. 0 HP / 0 SpD Shaymin: 160-189 (46.9 - 55.4%) -- 69.5% chance to 2HKO
And that's another point - other things like Garchomp can fire off base 100 or even base 120 STAB moves, while Aegislash is forced to deal with a base 80 move or even base 40 for the physical KS+SD sets. (Yet another important difference is that super low speed, meaning Aegis doesn't get to clean up switch-ins before they can move like MegaKhan could. Yes, he's got SS, but it's fairly weak especially coming from the mixed set.)
To make a more analogous situation, using sucker punch on anything because you predicted it to attack when they actually end up going substitute is also disastrous.
Quite frankly, the risks of misreading your opponent are almost always big especially when playing vs. offense, I don't see how they are so much bigger with Aegislash to make mid-game decisions involving him so fundamentally different from such decisions involving any other pokemon that they could actually make a pokemon deserve a ban.
Again, overpoweredness is another issue altogether; although I don't really think he's overpowered, I definitely think he's perhaps just under the line. But this 50/50 stuff is nonsense.