"Coverable ground"? Comparable to most pokemon Aegislash really is a one-trick pony. Its only viable special moves are Hidden Power, Shadow Ball, and Flash Cannon. In using its priority move, it opens itself to be OHKO'd by almost everything in the metagame. It needs an SD under its belt before it can really be threatening with Shadow Sneak, and because it is so slow can easily be crippled with WoW.
A Careful Hippowdon with no SpD investment is only 2HKO'd 11% of the time by a Max SpA Quiet Aegislash. Note this is with no SpD investment, which is rare. An Impish variant running a minimum of 104 SpD EV's is only ever 3HKO'd after leftovers. Also note that commonly, Aegislash will run Brave and have Shadow Ball for pokemon such as Treveneant and Gourgiest - the physically oriented set plays far more effectively than a special one.
Shadow Ball requires dropping another moveslot. Generally, you either drop Shadow Sneak or King's Shield - either way it opens Aegislash up to being countered effectively. On the special side, pokemon like Goodra can tank hits (while resisting Sacred Sword) and KO with Flamethrower.
Honestly, there are so many pokemon that can stop Aegislash that are common in the current metagame that it really shouldn't be anywhere near being banned. Heck, Klefki can beat Aegislash 1-on-1 with Foul Play.