Well, I tried three. And I don't like it.
Very unnatural to use. I specially don't like the swipe thing. But that is just an opinion. I can understand why people like it.
The most I dislike about 3rd P Keyboards is that you have to give them full access. And I don't know what they then collect on data etc.
Swipe on swiftkey won't work unless you activate full access. And It feels so un-iOS to have brand names on the spacebar or weird setting buttons on top of the keyboard.
I understand that people want this. But I think it will take away the easiness of using iOS. For me, this was a very short experience with third party keyboards on iOS. I hope extensions will work better...