The analysis of personality under a humanistic/existential theoretical perspective is different because it entails that our personality is formed as we continuously meet our essential needs, which include basic physiological needs as well as deeply-spiritual and ethical needs.
Based on this paradigm, it is safe to conclude that the universal traits that, according to theorists such as Allport, Cattell, and Maslow, all humans share, are all created through the most essential meeting of the most basic of needs. Hence, since most humans get fed, sheltered, hydrated, etc., chances are that there is at least a basic number of traits that we all share accordingly.
Unique personality traits are also a result of the process of meeting needs. However, humanistic/existential theory deems the "unique" and "salient" personality traits differently. This is because, under this paradigm, whenever needs are not met and the individual experiences a lack of support, the "unique" results are often negative and the final outcome is pathological behavior.
Hence, the assumption under a humanistic/existential perspective is that, if all humans meet all of their needs, they will strive for fulfilling more sophisticated, philosophical, and spiritual needs, thus elevating us equally as a human race and universally; universality is the wanted result.
Contrastingly, an inadequate balance of met needs will result in a deviant individual that will not be able to fit in society and will continuously be in search of meeting those needs on his or her own. As a result there is a risk of anti-social and pathological behaviors that could only be resolved once a balance is found.