They can vary in terms of the number of words that contain a given phonestheme, their frequencies, the strength of their association with the core meaning of a phonestheme (for example measured as an average of human ratings for all the words that comprise the given phonesthemic cluster) and the regularity of that association (what proportion of words in the whole cluster are highly related to the predicted meaning).
psycholinguistic data is to some extent ambiguous on how these features of phonesthemes affect their productivity, learnability and their effect on language processing, which could partly be due the methods being employed.