I’m about halfway through unit 3 of Duolingo’s Japanese course, after a year and a few months. I picked up a book on Japanese after 2-3 months of Duolingo’s “we explain nothing” approach, which was helpful, and picked up WK after about 8 months, which has been excellent. WK and Duolingo are now dovetailing in ways that are very reinforcing (e.g. DL taught me the word for “hospital”, as part of a section on how to handle being sick, then right after that WK taught me “sickness” and “institution” and I could feel smart about putting them together…). I appreciate the grammar that DL manages to weave in, so that you learn how things should sound and how the various formations go. It helped with learning the difference between tanoshimi and tanoshimu, for example, that DL shows tanoshimi always is followed by ni, to look forward to. WK of course doesn’t give much context, or even how the verbs/adjectives look in various conjugated forms, though it gets you through vocabulary very efficiently.
But with the goof-ups in the recent voice additions in DL, I would have been lost without already having been taught the on’yomi and kun’yomi differences from WK. Whether I could have handled that a year ago as a complete beginner, I don’t know.