Really interesting! I was wondering a few months ago how autocompletion on phones works for agglutinating languages like Finnish, but I didn't have a Finn to ask =/ If you wouldn't mind enlightening me, does it even exist for Finnish? If so, how does it work?
I can tell you based on Russian that it works poorly compared to English, esp. for verbs. I'd expect to be even worse for Turkish (which I used to know on beginner level), or Finnish.
When the word you are typing becomes obvious, it would offer 3 (3 is the default on my keyboard app) random forms that do not appear to take context into account, or if they do they're often wrong. You can either just finish typing, or choose one of those, erase the ending/suffixes and fix them.