Well... I was afraid about the game breaking potential of this new Star boost system but it so much expensive...
You need 680 Rainbow Shard to rainbowize a 5*. A 50 FG roll give you 22 Shard. So, you need 31 roll to turn your 5* waifu into a 6* waifu. In other words, it cost arround 1550 FG just to make ONE rainbow. Just by making these 31 roll you're almost certain to get at least one true legit rainbow. Maybe more if your name is Wutan. This new feature is really attractive but don't really worth it at the moment. I hope they will add other (free) sources of Rainbow shard in the future (I know you can by them with D-medal but once again, it's so expensive and D-Medal are already usefull for 5*6*salt ticket). Maybe by them directly with FG with more RS per FG?

On the bright side, it will not break anything or decrease the rainbow value. That's a good thing for the game (both for game mechanics and game benefits for FKG team).
On the dark side... if raising 5* to 6* is so hard and Rainbow Shard so hard to get, no one will bother with raising low rarity girls to at least usable 5*. That's a bit sad. It was really good to take 2* to 4* out of the shadows but at the end, it will just make 5* shine even more.

And about equip and skill slot lost, I guess it's better to wait for new rainbowizable girls gacha to get a dupe and use her instead of wasting your beloved 5* waifu full everything. Espacially if all thoses gacha give 12% gold for 3 FG instead of 6% for 5 FG.