Why is it difficult to classify many Sino-Tibetan languages precisely?

Classifying many Sino-Tibetan languages precisely is difficult due to several complex linguistic and historical factors:

• Substratum Influence: Many languages in the region may have a layer of Sino-Tibetan features superimposed upon an older, non-Sino-Tibetan foundation (a substratum). Determining whether a language belongs to the family often depends on a scholar's ability to identify this underlying foundation, which remains elusive for some groups like Karen.1
• Massive Convergence: Over millennia of intense contact, languages in the area have undergone significant structural convergence. This makes it exceedingly difficult to distinguish between ancient loanwords and genuine cognates, complicating genetic classification.2
• Lack of Data: A great number of languages classified as Sino-Tibetan are known only from limited word lists or lack sufficient descriptive documentation to allow for valid comparative analysis.1
• Absence of Historical Records: Except for a few major and minor literary languages, most Tibeto-Burman languages lack written texts predating the 20th century, hindering the reconstruction of the ancestral Proto-Tibeto-Burman language.3