Interesting question! I ran a quick search and found that there's an academic research body called "English Profile" (<a href="http://www.englishprofile.org/" rel="noopener noreferrer" target="_blank">http://www.englishprofile.org/</a>) that undertook a research to see what the most common words used by non-native speakers were in accordance with their proficiency (A1-C2). There are probably several such bodies but this kind of research would explain the word classification.

That said, difficulty really depends on the individual and the native language of said individual! I remember this one time a Spanish speaking friend whose English level was maybe intermediate suddenly used the word "regurgitate" :) I mean, for him it was the direct equivalent of "regurgitar" and in English that likely wouldn't be a B1 or B2 word.