After further tests we can confirm what is logical in some way: the iPhone and iPad hardware is similar powerful, so the iPhone operates as expected. In fact the question was already with the iPhone 4s, why Apple stiill limited this functionality to the iPad (the 4s was much more powerful than the iPad 2, so why did the 2 have USB Audio/MIDI, the 4s not? I did not get an answer, of course...).
So UFX, UCX, Babyface and OctaMic XTC can all be used with the iPhone as they can with the iPad, as long as the iPhone has iOS 7. We just need to find out if this feature is limited to specific models, maybe the older 4 is excluded.
Also the same restrictions apply that I noted in post 69 here. For example the BF offers 192 kHz playback without resampling, as long as the 2 channel mode is active and the app in use supports that. Multichannel record and playback is no problem as I just tested with Multitrack DAW (the problem here is to find a DAW that runs on the iPhone at all - for example Auria and Cubasis do not). But typical DJ apps, like Djay/2 and Traktor can now run on the iPhone and have separated stereo outputs via the connected CC hardware, for mix and monitoring/phones.