Mew can't learn Defog or Soft-boiled in Gen VI.
Soft-boiled isn't a problem, since Roost is an identical replacement in 99.9% of situations (unless you are deathly afraid of imprison swoobat/kyurem).
Stuff like Defog and Fake-Out being unavailable are pretty damning though.
I kind of wish there was an alternative to Smogon which focused more on battling with cartridges and covered a greater degree of formats, offering the next best alternatives and advice for situations where you really want to use a specific mon but don't have access to certain key moves or have slightly imperfect IVs.
Smogon is the best there is at what it does, but what it does is overly specific.