I've been a SwiftKey fan for many years. I've used it when I was on Android, and was happy when it became available on iOS. I've found it's predictions and many other features (the option for a separate numbers row on top) very good and productivity-boosting.
The biggest problem I've had with it (which is probably more of a Google/Gmail problem) is what happens when I try to search in the Gmail app. Somehow Google's predictions and autocompletion don't live well with SwiftKey. It literally drives me crazy.
I am also multilingual, on Android SwiftKey could have several languages installed and switching between them is easy. On iOS it's limited to only two languages, so I have Yiddish and English defined.
When Gboard originally came out, I wanted to try it, but it didn't have Hebrew or Yiddish at the time, so I dropped it, but I recently decided to try it out.
Gboard advantages:
1. Definitely coexists better (though not perfect) with Google predictions/autocomplete in the Gmail app search.
2. Can have 3 languages installed on iPhone.
3. Smart predictions (will come up with emojis by typing a term, will do calculations, etc).
Gboard disadvantages:
1. Won't remember/autocomplete email addresses, web addressed etc. (this seems to be a game changer for me, and I'm ditching Gboard over this for now).
2. Doesn't handle spaces as well (for example, if I type a word, and it's autocompleted predicted in the singular form, if I want to make it plural it's not a smooth process).
What is your favorite soft keyboard? What are the best and worst features?