The grammar translation method is seem as obsolete, but Duo isn't that. You don't learn rules formally (e.g. memorise explicit and formal rules on how to conjugate a verb in the past continuous tense, and what all these rerms mean) then apply them.
If anything, people constantly complain about how Duolingo just gives them sentences and doesn't give long explanations about the grammar, you just have to pick it up. Very modern.
People also complain about how duolingo has "nonsense" sentences, because it deliberately drip feeds vocab in similar categories which is actually the right way. You learn one fruit, one colour, one body part, etc at a time; so yeah occasionally you might get something like "tom has a purple apple on his nose" but there's a reason for this.
The only real faults with Duolingo is that it focuses on listening and reading, so you need to practice speaking and writing elsewhere. It does have an AI chat, but it's... kind of bad IMO.
And that most courses only cover a year or two of learning. And that there's very few languages. But if you want to learn enough to get started in more immersive learning, IMO it's fine.
And there's people who complain that they spend so much time metagaming to try to win the weekly leaderboard that they actually hurt their learning, but if you really need a cartoon owl to give you a cartoon gold medal then maybe you shouldn't blame the app ...