A-landing with Marth and Lucina isn't really beneficial. The only moves they have that can auto-cancel before the hitbox comes out are Bair, Uair and Dair, and you have frames 1 and 2 in which to do so.
Even so, you don't gain anything from getting a successful A-land. You're better off learning when to fast fall to auto-cancel as quickly as you can (Bair is really good when fast fallen and AC'd).
Uair's AC frame is relatively strict (our SH air time is 41 frames, and Uair's AC frame is 38>), but don't be fooled, as 14 frames of landing lag is still relatively low. Spaced at max range on landing, it's safe.
It's thanks to the low landing lag that falling Uair is a good combo starter/extender.