"Some of it" tells me that the uncountable noun can be divided or catagorized into "parts" and some uncountable nouns are more logically divided into parts or categories than others (for example, history, mathematics, music etc. can be catagorized) and therefore the "some of it" for music could be classical, pop, jazz etc. However, your "some part of it" could also be correct as melody, lyrics, rhythm etc. On the other hand, other uncountable nouns are not easily or logically divided into parts (for example, coffee, milk, snow etc.). So, to use your example question "Do you like this coffee?" , a logical answer would not be "Some of it" nor would the answer "Some part of it".