To be fair, there are few things that Mega Sableye does have over the other three magic bouncers:
1.) MUCH better typing
2.) Much better physical bulk in conjunction with Will-o-wisp (HP still sucks though)
3.) Isnt weak to ghost and dark moves (Mega Absol has this too but its frail as shit)
4.) Access to reliable recovery (Xatu has Roost but lol Xatu, and Morning Sun is never used on Espeon)
5.) Priority Taunt/Will-o-wisp/Recover/Nasty Plot prior to Mega Evolving
6.) Can maintain an offensive presence without sacrificing its bulk thanks to Foul Play
7.) Access to STAB Knock Off and Shadow Ball (Mega Absol has the former)
That 4 Moveslot Syndrome and possibly mediocre Sp.Def is going to be its undoing, though.