I've been following the Fleksy app and testing it with VoiceOver when new releases come out.
After playing with the latest version in the app store, I think it is worth giving Fleksy another try. I know many people were discouraged when it was released as an OS-wide keyboard in iOS 8. Yes, the app wasn't very useable with VoiceOver in the first few versions. Some of the issues were with iOS itself and some with how Fleksy was implemented.
As the Fleksy team has worked to both update the app and include new features, however, I think it has now gotten to the point where it is pretty useable with VoiceOver.
I can easily type text, delete words, choose new words, switch to the numbers and symbols keyboard and back again, etc.
Also, one can now use either the predictive typing or custom typing by moving one's finger around the screen until the desired character is spoken and then the desired character is typed after one releases one's finger.
Anyway, I think that, as they have been working hard to include features for others that will make the app more popular in the app store, they have also been slowly but surely addressing some of these accessibility problems that have popped up. So, I'd be interested to know what others think of Fleksy now. I personally think it is working pretty well.