It's the worst route, to be honest. It's the shortest, it leaves some very obvious hanging threads, and making Rhea go from zero to sixty with her evil turn was pretty laughable. Especially since all she wanted was to see her mother (you) again.
Rhea going from 0 to Crazy is plenty believable. The first half of the game is constantly implying that she's messed up, and later revelations about what she's done throughout the years reinforce that.
The problem is that Edelgard literally decides it's time to attack the Church less than a month after your merger with Sothis, giving Byleth zero opportunity to know more about her heritage or what exactly she is. Then she decides she needs to desecrate the holy place, attack her former professor and fellow students, and shows up with a psychopath that makes Hubert look down-right friendly. Not to mention she had been teaming up with Those Who Slither In the Dark before hand. And most of the suspect shit that the Church is actually guilty of goes unrevealed in the BE route. So like all we have about Rhea being a bad guy in the BE route is that Edelgard opposed her and she's a zealot. Like so much shit (good and bad) about Rhea gets hidden in that route that it's such an incomplete story. All we get is "Rhea is mad that her mother's new host defied her." ---
TheRock ~ I had a name, my father called me Blues.