On normal, in the endgame.
Best - I have yet to encounter a problem that can't be solved by throwing Diamant at it, especially paired with Leif. Ivy, Rosado, Chloe, and Merrin are also standouts.
Worst - I haven't used Etie, Jean, Clanne, Anna, Lapis, or Citrinne since the start, so I guess that counts?
Disappointing - All but two of my time rewinds were because Alfred isn't nearly as durable as I keep thinking he is, and he isn't really keeping up (the other two were just because Chapter 17 is . . . fun). It also feels like I'm constantly babying Alear so that he can stay relevant.
I also agree with the sentiment that it feels like the opposite of the standard Fire Emblem unit curve - it feels like the units you recruit keep getting better the later in the game you go. Even with the extra investment you give them, nearly all of the early units apart from Celine and her crew have been outclassed by later-recruited ones.