worn vortex
Let's be honest, Rebirth is really doesn't worth the farm, juste to have the stat up a little, a costume (if you have the ring) and a anima, the "upgrade" for the spell cart are just useless
Do you remember how meiling was bad before (she was the worst character), now look at her how strong she is, why just do the same thing with the rebirth it would worth 10 time more the effort and resouce spend
surreal raven
Me with RE 
in lv120 ds reimu.... Xddd
dire plank
i think L1 Reimu got something similar in JP and global got her strengthened
slow stratus
global wasn't responsible for any of the buffs, they all came from JP first
stray path
First of all, saying Rebirth "isn't worth the farm" is a very weird statement because you don't have to put in any effort to get a Rebirth. They added a skip feature so you can literally just click Skip once per day for two weeks or less and get the upgrade. So I don't think it makes sense to say it's not worth the effort
That being said, some Fantasy Rebirths are indeed quite underwhelming, but on the other hand there are many that can be very powerful if used correctly. Units that get large bullet line upgrades like Lily White and Kanako can deal absurd amounts of damage, especially in Arena where buffs aren't as powerful. Patchouli in particular got a huge damage buff and a bunch of accuracy buffs, so her turn 1 damage is actually quite good these days. We definitely still get duds (cough, Sakuya) but a lot of Rebirth units are very usable
fresh agate
I agree that most FR units have trash changes for a very "long" 2 week process to gather their mirrors and materials to upgrade their abilities,just for the new decay mechanic and their already HORRIBLE elemental bullet spread to make the unit be even worse.
Hitting twice as hard is worth almost nothing if all i hit is elementless damage(im looking at you,alice)
frail osprey
If rebirth doesn't cost 14sp, then I'm sure people will gladly (ahem) marry their favorite units, but due to the cost and the tedious process of upgrading the character again, not everyone wants to do that.
I kinda get about the cost (rebirth was made before those shiny new stuff like efes/rfes or even retro was a thing), but I still don't know what is the point of revert their level to 1. If I just rebirth my unit, at least let me use her fully to pay back for all the hard work I have to do.