I'm about 50 hours into this now. I find the difficulty jumps significantly between some levels. Some of the units are just too useless to to carry on with. The flying units especially. Just let them die and don't bother wasting exp on them.

Looks like Nintendo did indeed screw up the shipments, time will tell if they rectify this or plan to go the route they did with the Wii software and physical copies become rare andd expensive within 18 months of release with Nintendo halting production.

The actual eShop gives them even more reason to do so.

You'll always find this in FE, really. Some units (especially Pegasus Knights and Healers) are pretty pathetic, and I seldom use exp. on them. But other flying units, like Dracoknights, are certainly worth levelling up. 

Pegasus Knights are troublesome early on to get leveled up due to being physically weak and having exploitable weaknesses with archers and wind magic but later on their mobility works quite well and add in the fact that you can now change their class once you get them above level 10 with Second Seal and you can change them into another class. I also definitely recommend the Dark Flier class as having a mobile magic user and one that gets a whole second turn once they kill a enemy once they have the Galeforce more then makes up for the early trouble.

Well take Cordellia for example.

She is a Pegasus Knight, but can be reclassed to Mercenary. Do that and then promote her to a Bow Knight. Level up Bow Knight to level 15 and you get the skill BowBreaker "Hit rate and Avoid +50 when the enemy is equipped with a bow"

Now reclass her to a Dark Mage and promote to a Sorcerer. At lvl 15 she can now learn TombBreaker "Hit rate and Avoid +50 when the enemy is equipped with a tome"

Then you can reclass her to Pegasus Knight and Promote to either of those Pegasus positions, and you are left with a Flying unit on a Pegasus that is granted +50 Avoid versus BOTH tombs and bows.

now sure that is a lot of leveling, but just saying. you can make her pretty damn insane.

I think her daughter has the same reclass options. Plus the carry over skill from the mother, you could shorten up the duaghters needed to level by having bowbreaker or tombbreaker transfer to her.

Hmm, I can only speak from experience from previous FE games (where the only good Pegasus Kinght was Elincia, and that was because of her sword Amiti and the fact she could use staves), but that actually sounds like a good plan. Sure, it's a lot of skirmishes, but it's worth it in the end!

I find that healers are essential and they build quite quickly just by healing, then they turn into awesome war clerics.

most useful pegasus rider was that girl (from Ike's Adventures!) that can summon up to 6 royal guards! Oh she was my top 10 in both games!

The only flying unit I have left is a dark flyer. I turned one into the thing that looks like a war chicken and was happy when it died because it looked so ridiculous. I find the mobility of limited use since you can't move them outside the well guarded pack or they will die.

I guess so, but I always find healers to be too fragile, and not being able to fight back for a long time with low speed makes them an easy target for attacks, and tend to be the cause of much frustration.