Why is it difficult to draw an exact boundary between vowels and consonants?

In phonetics, drawing a precise boundary between vowels and consonants is difficult because there is no universally accepted, rigorous definition that perfectly separates the two classes.1

Traditionally, a vowel is described as a sound produced with minimal obstruction in the vocal tract, allowing for a relatively free passage of air, and characterized as being syllabic. However, this description is considered unsatisfactory because linguists have yet to formulate an adequate, objective definition of what constitutes a "syllabic" sound.1

The ambiguity is further complicated by the existence of sounds that bridge these categories. For example, semivowels (such as the y in "yawn" or w in "walk") are articulated in a manner similar to vowels but function as consonants within a language. Additionally, while Chomsky–Halle features provide a framework for classifying sounds using binary oppositions—such as "consonantal," "syllabic," and "sonorant"—there remains ongoing debate and difficulty in defining the phonetic properties of these features to create a definitive, mutually exclusive boundary.23