My usage case is probably a very edge case, but my biggest complaint with Android 3rd party keyboard ecosystem is how hard and inconsistent is to switch between third party keyboards. I like SwiftKey, but I use about three languages daily and SwiftKey is not good enough for two other languages I use (Japanese, Thai), so I have to use other keyboard.

Switching between multiple keyboards in Android is totally pain. This was over a year ago, but back then I have drag down the notification area, choose Change Keyboard, select keyboard I want to use and go back to typing (maybe optionally switch the layout to the correct one before that) instead of a single button press. Some keyboard do provide a way to switch to other keyboard, but they're mostly non-obvious (e.g. in Google Japanese IME, you have to long press the layout switch [A/あ] button for few seconds, then choose "Change keyboard"). Even though most non-English keyboards also provide English layout, but most of time they're simply half-baked or just downright bad.

I don't know how Apple designed their third party keyboard API, but I truly hope they have some kind of guideline regarding the keyboard switcher, and that third party keyboards in iOS are per-language so all non-English keyboards out there don't ended up implementing a half-baked English keyboard. Either way, I'm really happy to see SwiftKey coming to iOS, even more so when my favorite Japanese IME (ATOK) announced they're going to release a keyboard for iOS.