This is tangentially related but:

I’m learning Japanese. I pay for private lessons. However, I studied Japanese on Duolingo for 100 days. I can say a lot about Duolingo, but I’ll just say this: I felt I was getting diminishing returns as I kept going. I wanted to keep learning Japanese, so I found a service and a native Japanese teacher and began taking lessons once a week. I’ve been at it for about 3.5 months and I’m just about halfway through their recommended textbook.

It’s related because I was studying Japanese for a long time on Duolingo (the weak tool). Discovered that the method I was using was no longer working for me, so I upgraded and got a better learning method; paying for lessons (the durable tool).