I'm almost sure autocompletion in Finnish (or Estonian or Hungarian) is borderline useless, as the chances to write a word that wasn't ever written before are quite high.
But even with more "sane" grammars these models barely work. When I write in Spanish, often there are verb forms that are missing that I have to type fully.
For example, in Spanish, all forms are on conjugation lists, but it can be tough to find a set of corpuses that covers all. I know that a 2016 Spanish Wikipedia dump I played with covered about 20% of all verb forms present in rae.es.
Then on all those forms (I'd say about 16-18 tense/mood/aspect forms are in common use) you have to take into account enclitic/affix pronouns, and the whole thing goes awry quick.
"Comámosnoslas" - If you search on Google, there are only 9 results[0], this post likely to become the 10th. But it's a completely normal word a Spanish speaker may use and will understand.
comamos ("let us eat") nos (emphasis "for ourselves") las ("them", feminine). "Let's eat them!" but with emphasis lost in translation.
E.g. usage "Hay 3 pizzas, ¡comámosnoslas!" . This sentence, funnily, is properly translated by Google Translate into English, but it tries to correct it to "comamosnos las" which is absolutely broken Spanish.