Well lets take a look.
Avariel:
The Winged Elves seem a bit weaker than Aarakocra at first glance. But then you remember that they are still an Elf, which means that they will not suffer the problems of being a Birdman, and get the basic elf package (which is a good thing to have). However, if you are allowing flying races in your game, it's not mechanically stronger than the winged Tieflings, though the peasants will probably be more amiable to them.
Grugach:
Strong elves. The bonus in str isn't likely to temp anyone outside of the odd elf-barbarian. The weapon proficiencies exist to help out the Druid, Sorcerer, and Wizard. But the real windfall here is the free Druid Cantrip. Druids have a very nice selection of cantrips that lots of characters will want in on, though using wisdom for the casting stat is going to put some breaks on them.
Sea Elf:
The AquaElf. That's basically it actually, outside of those water-focused campaigns they aren't anything to think about. Constitution is always good. Trident proficiency isn't any better than spear proficiency unless you happen to find a magic one. Nobody uses nets. The only characters who would really want to use hand crossbows are the ones which have class support to back it up(and thus proficiency with them). That leaves "talking" to swimming animals, swimming, and breathing underwater as the draws here.
Still, it's nice to have stats for them now, my Pirate game was missing them.
Shadar-kai:
These things used to be Human-ish, and masochists. Odd that. Now they are half-undead goth elves or something.
As of right now, they are weaker Eladrin (the UA kind). However, they are clearly more geared for the Paladin or Hexblade playstyle (fitting I guess, given that they serve the Raven Queen), teleporting into the fray and getting resistance to damage while doing so.