I'm frustrated that they established Machu and Nyaan's character arcs but didn't finish them. Machu never found her true calling and Nyaan never really faced her traumatic kill-triggers.
The Shuji romance was so half baked it wasn't really a final reward and just felt like tying up loose ends.

I love them a lot and am enjoying all the fanart and comics of them but I'm sad that the show didn't treat them as main characters in their own show.