I'm a newish learner, using Duolingo and Kwiziq mainly, watching a handful of French shows and frankly obsessing over the French variants of Disney songs. I know this isn't helping me learn nearly as quickly as lessons at school once did. Admittedly, I'm not far through yet. I've just reached the A1 bracket on Kwiziq and that's mostly muscle memory from school, combined with a good bit of confusion over the whole 'qu'est-ce que' nonsense. I still forget how that works every ten minutes or so.

Duolingo seems more of a supplementary tool to be used for background practice and maybe to increase vocabulary to some degree rather than for actual in-depth learning. Without other resources, there's probably only so far you can go.