Why is the classification of Mongolian languages controversial among linguists?

The classification of Mongolian languages is considered controversial because no single scheme has achieved universal approval among linguists. Several factors contribute to this lack of consensus:1

• Subclassification Challenges: While Mongolian languages are often divided into western (e.g., Oirat, Kalmyk) and eastern (e.g., Buryat, Mongol) groups, many contemporary scholars no longer view this as the primary genealogical division. This is because these groups share numerous features, such as the loss of initial /h/ and the reduction of vowel sequences, suggesting a more complex historical relationship.12
• Dialect vs. Language: There is ongoing disagreement regarding whether certain spoken varieties, such as Buryat, should be classified as distinct languages or as dialects of a broader Mongolian language. Furthermore, while there is general agreement on identifying specific dialects, their allocation into larger groupings remains a source of debate.2
• Altaic Hypothesis: The broader classification of Mongolian languages within the Altaic language family is itself a subject of significant controversy. Most scholars now reject the hypothesis that these languages share a genetic relationship, though some argue the category remains useful for describing areal similarities caused by geographic proximity.13