Ignoring all the rain fuckfest...Max defense Unaware Clefable is not meant to wall everything, only a slew of particular threats. Citing special attackers as mons that threaten it to support dropping it is utter bullshit because walling Special Attackers is not even what max defense Unaware Clefable is supposed to do. Hint: Mega Sableye is not meant to switch into Fairy-types; yet, with your logic, we should drop it because it fails to beat Azumarill / Mega Diancie / Mega Gardevoir.
The problem with this argument is that fairies affect Mega Sableye to a much smaller scale; either that or a very strong hit, since Sableye is bulkier than Unaware Clef on the special side, and not that much frailier on the physical side. Any decently powerful special hit can 2HKO Clef with little problems.
252 SpA Life Orb Thundurus Thunderbolt vs. 248 HP / 8 SpD Clefable: 203-239 (51.6 - 60.8%) -- 93.4% chance to 2HKO after Leftovers recovery
252 SpA Life Orb Thundurus Thunderbolt vs. 252 HP / 144+ SpD Mega Sableye: 133-157 (43.7 - 51.6%) -- 10.9% chance to 2HKO