Writing in 2004, John Huehnergard notes the great difficulty in establishing cognate sets across the family.
ɹˈaɪɾɪŋ ɪn tˈuː θˈaʊzənd fˈoːɹ, dʒˈɑːn hjˈuːhnɚɡˌɑːɹd nˈoʊts ðə ɡɹˈeɪt dˈɪfɪkˌʌlti ɪn ɪstˈæblɪʃɪŋ kˈɑːɡneɪt sˈɛts əkɹˌɑːs ðə fˈæmɪli.