I think that Buck depicts Wang Lung as a man capable of the greatest of successes and the most humbling of failures. Simply put, both elements are at play. Buck does not skimp on depicting both aspects of Wang's personality and identity. In the end, the reader is left to judge Wang as one who possesses attributes that are desirable in the individual, but also acknowledge that his failures should be avoided at all costs.
In terms of the successes, Wang's perseverance and desire to work harder than anyone else in order to make his vision come into reality is something that Buck draws out in an admirable light. Wang Lung starts at the bottom rung of the social ladder. Nothing is handed to him and nothing comes easy. Wang is the living embodiment of the opportunity ideology. His work demonstrates that the cosmic forces of the Earth rewards those who acknowledge their own subservience to it. The universe unfolded the way it was supposed to for Wang in his willingness to subdue his ego in the name of work. At the same time, he displays a healthy respect for money, never spending it to excess and always remembering how hard he had to work to obtain it. Wang patiently moves up the social and economic ladder of success, building upon advancement after advancement in order to find a sense of happiness and contentment. There is something heroic in one who has nothing and gains much. Buck knows this and is deliberate in such a depiction. Wang's rise to economic prosperity reflects the standard notion that there is both external and internal reward for working hard and paying one's dues. These elements have to be seen as part of Buck's portrayal as a success.
Yet, there is a flip side to this coin. Buck does not engage in hero worship. Wang is shown to possess some level of vices that really detract from his attributes. Wang is shown to disrespect women. It is difficult to see Wang as a success with his treatment of O-lan. Interestingly enough, Buck is deliberate in her parallel development. Wang's successes end up developing and growing once he marries O-lan. However, his treatment of her is abhorrent. Whether in small instances, such as him taking her jewelry for his own economic development, or in the larger transgressions such as standing by and essentially forcing her to kill her second daughter, or taking another wife and relegating her to servant status, Wang has to be seen as a failure as man and human being in his treatment of O-lan. The same appropriation with which he used to acquire economic success is seen in his treatment of women, as in the manner he appropriates Lotus and Pear Blossom. Even if one concedes that Wang is operating well within social contexts, the degradation of women that he continually displays are elements to consider him an economic success, but a personal failure. Once again, Buck shows these failures as reflecting how Wang has moved away from "the good Earth." Wang's failures as a man and husband are moments when he embraces his own identity as superseding the natural order of existence. It is in this light where he is a failure.
The ending of the novel shows Wang to be a man who has seen some level of errors in his ways. His desire to return to a more simplified life is an admission that he has lost his way. Buck makes it clear that we temper this loss of direction within our understanding of Wang. In this, one must make a conscious choice as to which is more persuasive. It might be easy to move past Wang's transgressions as lapses of reason. However, it really is difficult to see an abusive and disrespectful character as heroic. As a result of this, one sees Wang as a human being, equally capable of fable- rendering greatness and inflicting torment on other human beings. It is in this regard where one must make a conscious choice within the depiction that Buck offers of her protagonist.