Good collaboration requires being on the same wavelength, making work flow painless. The hardest thing is when that is not the case and everything requires a discussion and a degree of moderate coercion. Those types of collaborations don’t usually happen too repeatedly. But there are very different types of collaborations, as in the role one plays, (musician, writer, producer, mixer) and this dictates how you mentally approach the project and the level of control you take as opposed to compromises you make. I generally prefer to be left to my own devices and exchange audio files once I’m happy with the way the work is going, but with long standing collaborators, on more familiar ground, working in the studio together can produce faster/better results (but not always). Recording budgets are also a major factor to the work process as commercial studios are still extortionately priced and create a time pressure for everyone, therefore if you have your own, even modest, set-up (thanks to advances in technology) you can work for much longer perfecting ideas rather than going with snap decisions due to budget restrictions.
As far as I can recall there are no unreleased collaborations … maybe some session recordings but I’m not entirely sure. I do remember back in ‘82 the Takahashi live band (other members included Masami Tsuchiya, Hajime Tachibana and Haruomi Hosono) recorded a track which was intended as a single, and at that time Hosono and I also started to collaborate on some new tracks but busy schedules meant these projects were abandoned for more pressing commitments. Generally speaking, once I take on a project I like to see it through to fruition.