It's really tough to find weaknesses in Ruth's character. If one were to be scrutinizingly unsympathetic, a weakness could be that Ruth does not operate out of a position of strength. She finds herself amongst the cratered remains of Warsaw, struggling to make a sufficient life in the wake of one of the most horrific events in human history. She is weak in so far as her power is limited. That is about the most in terms of weaknesses that she possesses.
Overall, I see Ruth as a paragon of strength. Ruth is a transformative figure in a time where such forces were rare. Death and destruction were the norm throughout Europe in the wake of World War II. Poland was especially ravaged. When she sees Warsaw after the Nazis evacuated, Ruth is unable to recognize it because of the destruction it endured. Her strength is evident in this setting. Ruth assumes control of her siblings, as opposed to abandoning them. In this singular act, one sees an immense reservoir of strength. She is able to fully assume a role of shelter and control in a world devoid of it. She reaches out to find her brother and start a school for children in gutted out Warsaw. Ruth faces disrespect and marginalization from different corners and is unfazed by it. She represents a source of physical strength in how she is able to continue on and persevere throughout trying conditions. For example, Ruth does not show weakness in pursuing Edek. When he is not initially found, Ruth presses on to find him. Had she been weak, she would have abandoned the search for her brother or not commenced it, in the first place. Ruth's strength is shown in the commitment to her brother. Ruth demonstrates strength precisely because she does not give into human weakness of isolation and loneliness. She represents what can be in the midst of what is. This shows strength and a lack of weakness.
Ruth also represents strength in how she interacts with others. She does not allow weakness in the form of selfishness and mistrust interfere with how she perceives other people and her place in the world. This is seen in her befriending Jan. If Ruth was weak, she would have denied Jan's voice, marginalized him by isolating him from her family. Her example inspires him to overcome his weakness. In the pinnacle moment where he must choose between his own interests and someone else's, Jan is able to save Edek because of the example that Ruth has represented to him. Ruth's strength resonates in how she sacrifices herself for others, believing that what ought to be is what should be replicated in her actions. As becomes evident with Jan's transformation, such strength can be contagious even at points where human decency is in short supply.
Ruth's strength is drawn from the example of her father and from the New Testament. Ruth is perceived throughout the book as the embodiment of "courage, self-sacrifice, and greatness of heart." Ruth's power to heal is derived from her strength. In a world where human weakness was the dominant reality, Ruth proves herself as fundamentally different with her consistently revealed strength.