Swiftkey seems nice until you're not an english native speaker, which means you automatically write in 2 languages (native and at least some english). Now good luck having the keyboard guess which dictionary to use for every single word, which can even lead to mistakes if both suggest the same thing (capitalization).
I care about not having a wrong word in the middle of every other sentence.
And even that doesn't change the simple fact that typing on it is cumbersome. Swiftkey is a little better on average, but still not even close to an actual keyboard.
I have GBoard in three languages, it guesses the language from the first few words and then 'sticks' to it for the rest of the message.
Occasionally it guesses the wrong language if the first word or two are ambiguous, but by manually typing out a word in the right language it quickly adjusts itself.
Or if some languages are only used occasionally, you can just put them on a dedicated keyboard and switch with a long press of the spacebar.
The greatest flaw of GBoard is that it doesn't treat space as just another character that could be mistyped, but as a delimiter that's always assumed to be in the right place. This makes the error rate ridiculous.
Actually SwiftKey in multiple languages helps me write in the ones I am less fluent in, due to its autocorrect. I agree it can be annoying too... you have to be careful with activating languages, and I switch off the languages I am unlikely to use in the near future.
Still miss my BB though.
Edited to add: unlike other commenters here, I never use the glide feature in Swiftkey! I just use it as a better keyboard...
And the worst is if you write an English sentence with five common words, but the the AI is convinced the fifth one is actually a long and complicated German word. Our brave new world of shit AI keeps making me think about this goomics comic:
And even that doesn't change the simple fact that typing on it is cumbersome. Swiftkey is a little better on average, but still not even close to an actual keyboard.