Using the 3D Touch cursor in the DayOne app doesn't really work: the cursor disappears (who makes it useless) or is very slow. Whzt happens is that SwiftKey wastes time trying to predict or correct for each movement of the cursor, instead of switching this off during movement.
Another big problem is with the text entry on boardgamegeek.com, when adding a game to one's collection (requires registration): the web widget used reacts badly to SwiftKey's corrections, with a tendency to swap a word and the space that precedes it. More generally SwiftKey is confused about where it is in the text: it shows an incorrect word context (that does not represent the actual position of the cursor but something close instead). My guess is that this has something to do with registering the cursor position correctly, even when it moves or deletes characters.