I've been having fun using Aegislash in Ubers, and to laugh at those who think they sweep with a Geomancy! But in all seriousness, however, it has trouble against most of the other Ubers. Kyogre and Groudon are two good examples, Groudon hard walls Slash and can smack it with a EQ. Kyogre is faster than Slash naturally and a Specs Water Spout in the rain(even Timid nature) will always knock it out, even when Slash has SpDefensive investment. There is a new comer that does stop Aegislash in it's tracks, Yvetal. It's Dark Pulse will always 2HKO Slash in it's Shield forme, unless you get lucky with a low damage roll with a SpDefensive variant of Slash. There are three others that I'd like to list as well, Dialga, Darkrai, and Arceus. Dialga, what is usually glanced as a SR user most of times, is a problem to Aegislash. While the standard Defensive Dialga set in gen 5 is still a thing, it's Fire Blast will always 2HKO a 252 HP Slash, after Leftovers recovery as well. While it's offensive potential against Slash can be scary enough to switch Slash out, it all depends on Slash's set as well. If you are a variant that packs Toxic, you cannot touch Dialga at all, only if it's HP is below 20% after lefties, then you have a chance to knock it out. Even if you are a Sacred Sword variant you must have Atk investment to even have a chance to 2HKO, while Dialga can Fire Blast you and switch on the KS. Now Darkrai, Darkrai makes Slash cry. While it can put it to sleep, it will always 2HKO Aegislash-Shield after that. Once again, a SpDefensive variant can't really do much, even when following up a Sacred Sword with Shadow Sneak will still not knock out Darkrai. Now, onto Arceus. Almost all Arceus variants ruin Slash's day. While offensive Slash with Sacred Sword 3HKO's Max HP EKiller with lefties, EKiller get's a free opportunity to set up 2 SD's. While KS'ing a Shadow Claw sounds nice, but EQ is what scares Aegislash the most. Not even EQ, but Shadow Force as well scares Aegislash. "Oh, let's King Shield on the second turn!" Not the best idea since Shadow Force goes through Protect moves, so it'd be very unnecessary damage to Slash.
While I point out many of the Ubers that stop Aegislash, but it's capability alone to stop Xerneas is a great reason why to use it.