There is, understandably, a lot of concern about that full access warning dialog in iOS 8 when you enable a new keyboard. The biggest fear is that apps will log your keystrokes in hopes of delivering better service but at the same time collecting every word you type. Add a database of all your keystrokes to a company searching to be acquired and suddenly things get really uncomfortable. Getting full disclosure from app developers of how much data they collect and what they do with it is going to be essential for me in determining which third party keyboards make the cut.
I published a screencast last week of the TextExpander keyboard. Smile, its developer, has already explained how they use data and it is, thankfully, very responsible. No keystroke data is transmitted to any servers. They keep a small cache of keystrokes only long enough to determine if you've triggered a snippet and then they dump it. You can learn more from Smile's blogpost on the subject. I hope other keyboard developers follow Smile's example.