Why isn't Armads dragon-effective?

One of the many questions that probably won't be answered.

yet falchion has about 395 different effects built into it

Because it would make Hector Tier Most F***ing Broken Unit in the game. Actually, that's probably not true but it would shake the metagame a whole ton.

The Armads wasn't effective against dragons in FE7, but that's mostly because there is only one sole dragon in that game, that being the final boss. It is effective against dragons in FE6 though...
Makes you think.

There's always CYL Hector but doubt it

And? Binding Blade was 2 range, had all phase defense boosts and a super vulnerary effect. They can't include every effect in the game so they had to pick and choose. All three Armads have enough effects as it is.