Duolingo Spanish is Latin American - so pronunciation is all wrong, plus some differences in vocabulary. Not helpful if you want to learn "proper" Castillian Spanish.
My pet gripe is that there is no option for English English rather than American English.
Doubly difficult when you have to translate toilette to restroom; football to soccer; automne to fall; match to game; and so on.
For me, the gamification of it keeps me coming back every day. I might not ever be fluent in Japanese, but I can ask where the toilets are, two beers please, is the beach hot, where are the police, etc. But I'm not moving there, it's just something to keep my head learning. :)
Apart from their widespread use of AI, they also don't use widely accepted teaching techniques for language learning, meaning that it's basically just vocabulary memorisation - useful as an adjunct to learning, but can't take you to any level of fluency.
Judging from my experience, it's great if you want to talk with viking cats who speak Estonian about a wizard Finn who sits up in the tree because he wants peace and quiet while listening to the music that swings like a moose.
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Doubly difficult when you have to translate toilette to restroom; football to soccer; automne to fall; match to game; and so on.
Not much use for real life scenario though.