What would be the weaknesses of this team?

Seems pretty weak to Reinhardt. Amelia is the only one who can survive a round from him. You could give her DC to deal with him effectively

I'd switch Amelia with Hector for a mixed team. And run either Fortify or Ward Armor on Effie so Hector can survive Reins and kill them on counter. Male Robin is better than people give him credit for, especially a +spd -res IV like my own. So he can take on a lot himself. Lucina is a little bit outclassed by Ryoma and Ike, and soon the Black Knight for that slot. He's actually almost as fast as neutral Lucina and has just as much attack, but with the distant counter advantage and access to Armor buffs (especially Ward). You might consider going all Armor Emblem with Hector, BK, Amelia, and Effie. That has potential.
Build recommendations? How much detail do you want? I will say that Robin should have Triangle Adept, natural Raven tome, and swordbreaker 2/3. Hector, just the standard Bonfire, Vantage, whatever C and assist you prefer. Amelia, QR 2/3 and Bonfire, otherwise base kit + preferred assist. Effie, Brave Lance+, her base kit, preferred C and Assist, and your choice of Moonbow, Luna, or even Sol or Aether. The Black Knight has a variety of amazing builds, so maybe pick the one that covers for your team best. Me, I'm thinking QR3 + Aether for his vital slots, Steady Stance, Ward Armor, and Swap/Pivot. But you've got literally like 30 great build options with him, so take your pick. Lucina, Fury, Vantage or Desperation, threaten spd or a hone, depending, Luna or Aether, and Reposition. Hope this helps.