I personally agree 99% with the Historical facts and Chinese linguistics that mention that both Cantonese and Hakka dialects are descendants of the original Han Chinese and that both Cantonese and Hakka preserve Middle Chinese [Sui, Tang and Song Dynasty Chinese] the most compared to other Chinese dialects.
When i mention Cantonese, i do not mean Guangzhou Cantonese [Standard Cantonese], but all Cantonese dialects like Taishanese, Zhongshanese, etc. Most Cantonese variants preserve the "i" vowel like the word 你 [Ni], 四 [Si], 十 [Sip], etc, except for Guangzhou Cantonese that evolved from i -> ei Example Ni -> Nei, Si -> Sei, Gwok Ki -> Gwok Kei (國旗).
I will use Taishanese as the representative of Cantonese, because it preserves the vowels better. Hakka and Taishanese both are similar in 3 ways:
1) They preserve alot of the old Chinese vocabulary words that are also found in Sino-Influenced languages like Japanese, Korean and to a lesser extent Vietnamese[there are a few words that are similar from my observation compared to korean and japanese]. Words like Ngit Bon (Japanese say Nip Pon) = 日本, Gai (Japanese say Gai) = 街
, Sai (Japanese say Sai) = 西, Ngi (Japanese say Ni) = 二, Ngin (Japanese say Nin or Jin/Korean say In) = 人, Nam (Korean say Nam) = 南 or 男, Sam (Korean say Sam) = 三, Hee (Korean say Hee) = 氣, Fi Gi (Korean say Fi Gi) = 飛機, Gim (Korean say Gim or Kim) = 金 and so on.
2) Both Taishanese and Hakka preserve the initial and final consonant. Both preserve the initial consonant Ng- (Example: Ngin = Person, Ngowt = Moon, Ngit = Day, Ng = 5, etc) and Final consonant -t, -m, -k, -p [Example: Yit[-t] = 一, Sam[-m] = 三, Luk[-k] = 六, Sip[-p] = 十].
3) Both Taishanese and Hakka preserve the "i" vowel. Example: Ni [你], Si [四], Mi Gok [美國], Sing Ki [星期], Gwok Ki [國旗], Pi Fu [皮膚], etc.
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