It's a good idea in principle. Personal reflection is key to a rewarding life. Over a period of years, behavioral and thought patterns emerge, both postitive and not. Having a record allows for recognition of those patterns and affords the opportunity to reinforce, modify or end them.
It's the execution that's flawed. If it only works on iOS, typing has to be done on an iPhone or iPad screen, which is limiting and slow. Not to mention, bad for the thumbs! If it works with voice recognition, that creates a new set of issues. We speak in the moment, for the most part without the benefit of verifiable or even reliable memory. When words are written, we have the opportunity to think about them before we write them, to refer and compare them to our past experiences and how we processed them at the time, all laid out for us from contemporaneous notes taken at the time they occurred. Only then do we proceed with what has happened since those previous notes. For anything more than a Cliff's Notes condensed version of events and experiences and our reactions to them, a physical keyboard, or better yet, a pen and notebook, are more important than portability.