Yes. There are channels recreating popular songs in garageband on iphone (with no additional paid plugins).
Physical knobs and buttons feel good, OP-1 also forces you to commit to specific audio and puts some limitations on yuu which may help with creativity.
You can create something amazing using just your phone. But to get skill to be able to do that, you likely need toys that keep you inspired and make you want to play all day.
The iPad will work alone, and if you're just starting out it's fine. But it's best to add a tiny MIDI keyboard to the iPad. For extreme compactness, the Korg Nanokey 2 works well for me. It has nontraditional "button" keys, but so does the OP-1.
The iPad's touch screen is usable, but its velocity sensitivity isn't nearly as good as a hardware keyboard.
I have zero music talent or experience, so let me ask: Would there be something missing with a quality soft synth on iPad with GarageBand?
The hardware buttons, sure, but is it possible to write a "serious" song with such a setup (iPad, paid synth app)?