Total anecdata but with a combination of Duolingo and a French wife I've learned conversational French much faster (3.5 years) than I learned conversational Dutch with Dutch friends and books (5+ years).

So I guess I agree that while Duolingo alone isn't enough to get good at a language I have been frequently amazed by how it "gets" where I'm at on my learning journey.

Especially recently I've started taking the occasional tests it provides and that seems to change my learning journey in a convincing way.