The main criticisms of Mio seem to be that she is "weak" and "illogical". However, she is actually a perfect depiction of a young adult who was abused through her entire childhood and now suffers from some degree of PTSD, fears of abandonment, and an anxious-avoidant attachment style. This in itself, if the character shows no progress, can make for a very brutal watch -- however, Mio is shown to gradually improve by reflecting on herself as the show progresses, and she makes tremendous progress in the 12 episodes. Some people have complained that it feels like the ML has swooped in and "saved" her, but to me it never felt like that -- instead, the scenes conveyed that experiencing true human care for the first time in a long time from multiple people made her reflect on herself and what she wanted from life. They both care for each other, and it shows through their actions as they stumble forwards figuring out how to communicate with each other. The interesting thing here is that ML does not make any grandiose gestures or dramatic proclamations -- instead, he slowly gets to know her as another person and tries to move forward in communicating with her to support her through overcoming her traumas. Sometimes, human partnerships can be as simple as finding peace in coexisting with someone, and I think this works well with the fictional time period that this anime portrays (with arranged marriages being the norm, and couples developing affection for each other by being live-in fiancees). (Side note: we have a very diverse female cast in this series, so it is a nice contrast with Mio so that the show is not saturated with the weak women stereotypes).
The criticisms of the plot include that it is predictable, world-building is poor/uninteresting, and "nothing happens". For the most part, I agree. However, a predictable plot does not necessarily mean bad storytelling -- the events unfold similar to the way they do in other stories, but you watch the characters respond to them in ways develop their relationship. The choice of magic in this world felt a little shoe-horned, but it was not offensive enough to dislike. It was integrated well enough that it felt natural as part of the setting, and provided a good tie-in for the overall plot points. If it feels like nothing happens during this anime, that might mean it's just not the right type of media for you. This story does not revolve around action, it revolves around the intangible changes in people and their relationships.
Overall, I found it beautiful.
