[yorwba]

The paper cited for the 68.3% figure http://web.cse.ohio-state.edu/~lin.3021/file/SEC15.pdf says that third-party IMEs are also "very popular" in Japan and Korea, though they do not cite any statistics. (Their statistics for China are from 2014. The paper was published in 2015.)

Chinese orthography is just as standardized as Japanese and hanzi/pinyin dictionaries are not harder to maintain than kanji/hiragana ones. Some people have trouble with sound distinctions in Standard Mandarin that don't exist in their speech and enter incorrect pinyin (e.g. z instead of j or zh), but that can be treated as a typo, the same as phonetic misspellings in other languages.

Support for dialectal variations is essentially nonexistent in mainstream IMEs. People who want to use varieties other than Standard Mandarin would have to use shape-based input (including methods that decompose each character into smaller parts, like Cangjie) or send a voice message. (There are projects to create IMEs for other Sinitic languages, like https://hanhngiox.net/ but almost nobody uses them.)

(Do any Japanese IMEs support non-standard dialects or even other Japonic languages?)