What does Growth rate do? what kind of benefit does it bring?
Growth Rate is your maximum level cap and the Arcana you can use.
If it has a Growth Rate of E you aren't using any Arcana and overall Level is going to be pretty low compared to S ranks.
Can't remember exactly but I think S rank was level 200? so it's 10 levels every rank below that.
So in essence with higher tier units you trading power for customization.
There are some higher tier units that have an "idea" or "kit" already set up for you that you would otherwise have to build on another unit with tradeoffs(like leader skills) but that can have their own advantage in that you can optimize more for their purpose.
Of course there are also units that are completely useless.
Here is an example of what kind of niche you units you can build if you actually look around.
Remember the humble Poltergeist? Kinda useless right? But it also has 2 item slots, that means you can put 2 Demon Explosives + 20 Self-Destruct from the title for a total of 80 with the cheapest cost at 2.
Of course it's not ideal since it has a bunch of deadly resist and Target Miss so it's a bit tricky to use.