YMMV but in my experience the most challenging projects were personal ones. Why not in the job? Because all the stake holders are interested in making the proje

YMMV but in my experience the most challenging projects were personal ones. Why not in the job? Because all the stake holders are interested in making the projects not too difficult or experiment, so they get realized. For my own projects I can make them as unrealistic as I wish. So of course the failure rate is also much higher than for my professional work. That way I got unusually proficient in certain technologies.