So, this has been on my mind for a while now. I personally think that the reason why FE can be so hit and miss with it's characters is because the cast of playable characters is so huge, which I believe can lead to an increased risk of inconsistantcy in character quality and character balance.
Now, the quality of characters is subjective, but I think that if future Fire Emblem games focused more on a smaller cast of characters, say 20 playable characters maximum, I believe that the writers will have a much easier time build stronger character development, as well as stronger unit balance.
But again, that's just me.