The mobile apps don't have grammar, but the web site does, for most skills in most languages. Also, grammar is easy to get from other sites.
The sentences are not taken from corpora, but edited by Humans, mostly volunteers. The courses are improved using user data. Some of the courses are indeed not the best quality.
If TTS is used (many courses use natural speakers), it was OK for me, so far.
Duolingo isn't perfect but I think it's the most efficient way to learn a new language from nothing up to a certain level (A2, maybe B1). If used correctly. You need to decide yourself how much time you spend on repetition vs progress. And yes, you probably need something else for full grammar explanation or Chinese/Japanese Characters. Though they are reportedly working on better Character exercises.