I use Duolingo to learn Spanish. It's very simple but the streak system has kept me making slow progress. I've only gotten to "Checkpoint 3" (of 7) but it seems to be working well - I don't speak fluent but I can usually read signs etc. in Spanish and actually understand them.
The main issue I can see them facing is: it's very simple, so easy to clone. And Duo has its flaws, so I wouldn't be surprised if of those clones are a lot better. I like Duo, I use it now, but I could switch to another service in 1 second.
The main issue I can see them facing is: it's very simple, so easy to clone. And Duo has its flaws, so I wouldn't be surprised if of those clones are a lot better. I like Duo, I use it now, but I could switch to another service in 1 second.