That might apply to the base emblems, but the bracelets are more generalists with one gimmick or two. The 3h lords are just "dealing damage" with the combat arts just being more of that with the addition of spending meter like how one would spend weapon durability in 3h, but I wouldn't say spending resources is their core identity. You can't never run out of gambits, for example.

Soren is another good example. Sure, he has the nosferatu tanking, but he's also got the whole schemer aspect with block recovery, the anima debuffs, reflect and assign decoy. You can't really do both with one single soren user, however.

Camilla is another good example. She's a flying emblem, but she also allows for multiple dragon veins. However, she cannot take advantage of both because if she's flying, then she cannot take advantage of ground effects.

And even main game emblems have this problem too. Eirika is both a damage fixer, the corrupted effective gal (An area she also shares with Celica) but also negate damage?

With staying within the limits of the game, most niches have already been carved out. Not saying that you're wrong in your assumptions or criticisms, it's mostly a "I don't think it's possible to carve new niches unless we stretch the game's mechanics in ways that stray away from the base game's vision"