I think it's pretty good. Alfonse's personal Sword isn't that great anyway, and Brave Sword+ works great with Death Blow. The Cons you mentioned basically don't matter imo, since Alfonse was always weak to those things anyway. Now he's gained extra roles in addition to strengthening the ones he already excels at.
I think Reposition is better for Alfonse, since he gets rekt so hard on enemy phase. Pulling back a mage or something is more useful than putting him in danger. Moonbow works better with Brave weapons because of the low Might, so yeah I think it's better than Draconic Aura. DA would be good for Alfonse's personal Sword.
Threaten Def would be good. It's the slot for support skills anyway, so it's not like there are really better choices.