That was probably the most disappointing final boss out of the bunch. Rhea going crazy really came out of nowhere and was pretty forced since it was never implied that dragons in Fodlan can undergo power incontinence like that. Even with the excuse of 'Thale's nukes caused it', it doesn't fully explain why that was the case as it was just an offhand comment, so it just comes off as a flimsy excuse to justify it (not to mention that Verdant Wind contradicts it). It does put the consequences of Rhea's action within the church on display admittedly since the knights and church workers also go crazy, but the thing is it's still not properly built up to either so it's still a lackluster way to use the thematic. As for the battle itself, it's just OK. The music is great, Funeral of Flowers is a beautiful theme for what they're trying to go for, but the battle itself was kind of lacking in oomph so to speak as I didn't really get a challenge from it. The beasts and other regular enemy types were easily dispatched and while the golems are threatening, it was easy to draw them out and defeat them, particularly since unlike in certain maps nothing boosts their stats, so it wasn't a difficult clear. All in all it's a pretty disappointing end to the route.
That also leads into my problems with the route itself: It's pretty underwhelming. The first and probably biggest issue I had with it is that it and Verdant Wind follow almost the exact same progression of their events, minus Gronder Field and the aforementioned final boss. I already mentioned my misgivings with the final boss, but the omission of Gronder Field, while it makes sense since we're a fourth faction and thus wouldn't play a part in a melee between the three lords, still glosses over it in a way that made me feel disengaged. Dimitri is stated to be dead, Claude goes missing, and the battle is said to be gruesome, but the problem with just stating those three without having any connection to the events makes it feel lacking. Dimitri I could forgive since he does get a ghost sequence, but Claude just disappears entirely despite being a major lord (in fact, he doesn't appear in the timeskip arc at all), and the lack of seeing or hearing much of the results of the gruesome elements removes a good amount of the punch that it had in the Azure Moon and Verdant Wind routes. It's a disconnect that while I understand why it happened, still hurts since it removes the feeling of war and loss, and it doesn't make up for it since none of the other students aside from Ashe or Lorenz appear in the route otherwise.
Also, I felt the Church and the cast were surprisingly underutilized. The first problem is that the focus shifts to Byleth as the leader and that comes with its own obvious problems since the character isn't made to carry the plot like that. Seteth does his best to carry scenes and he is a great character that I like seeing get that focus, but there's only so much he can do when the story doesn't allow him to take center stage like the three lords did. Also, the story doesn't really do much with the cast aside from the occasional chiming in on certain story events, but since the BE students are also there they're fighting for attention and none of them really get it. The only characters I felt really benefited in the route were Seteth, Flayn, and maybe Ferdinand and Caspar, the rest don't have as much to contribute. Also the church isn't really explored until the last few chapters after you rescue Rhea where they finally give you all the information that you want such as Byleth's past, so it doesn't really establish its own identity until you're nearly done with the story, and while that's partially the fault of repeating the plot points of another route I don't think its own elements allow it to stay afloat like Claude did with Verdant Wind. In the end, it's a route that I feel could've done more, you do get some revelations that were very needed and the characters who do get focus like Seteth do appreciate the chance to shine, but it wasn't enough.