First and foremost, no separate number row. Then, as if this weren't enough, the symbols are split into two different screens. SwiftKey also has two symbol screens, but at least those symbols that are most used are in the same screen.
Whenever I have to type a password (which is when iOS blocks SwiftKey and forces its own keyboard to come up), I spend more time switching between screens than actually typing the password itself.
Then, most diacritics are only available in the keyboard for a specific language. In order to use, say, Romanian and German diacritics, I must install a German keyboard and a Romanian keyboard and switch between them all the time. Whereas in SwiftKey if I hold a letter pressed I will see all the possible diacritics for that particular letter, be they German or Romanian or French or whatever.
There may be other quirks, but these are my main gripes. Especially the number row; that's a killer.