Imagine typing English by speaking. You can say "flower", but that might get written out as "flour". Some intelligence has to be implemented that picks the right one based on context, or give you the ability to correct it. That is where the complexity in east asian input methods come from.
(Yes, as English-speaking computer users we are very lucky. The exact sort of symbols that readers of English expect map 1:1 to our keyboards. Still kind of a pain on a phone, though!)
I'm not sure I understand your point. Without keeping up with the new memes, IME still let people type them (it's simply not as easy when IME does not auto-suggest the new combination, users can manually select each Chinese character).
Regarding "an informal ambiguous layer", are you implying there is something more fundamental/low-level than the Chinese characters used in communication? If so, what is that?
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?)
