What's so bad about Duolingo? I'll go into my criticisms of Duolingo for the Chinese language. I completed the entire Chinese duolingo course around 2018. I'm vaguely familiar with Duolingo for other languages, and I think that some of these criticisms apply to other languages.
1. Uses translation as a task. The main way that Duolingo teaches is by forcing the user to translate sentences between a source language and a target language. Sometimes this is made easier by giving the user a set of words and only requiring them to arrange them into the "correct" translation.
I think the intuition behind this technique is that people who are bilingual have some ability to perform translation. Thus, the idea is that one can gain understanding of a language by learning how to translate.
However, I believe this is mistaken, as being bilingual is something entirely different from having access to translations. The reason for this is that different languages do not communicate the exact same information. To give some simple examples: one language may require that every sentence specify a gender or a social role for the speaker, whereas other languages may leave this ambiguous. Some languages may require the speaker to specify time, whereas other languages will not communicate this. Additionally many words may have a straightforward meaning, but will still carry subtle implications or secondary meanings. In English, "senior" and "old" can both refer to an elderly person, but "senior" has more positive connotations and "old" has more negative connotations. A different language is unlikely to carry these same words.
2. Assumes that there is one valid translation (or just a handful of valid translations). Thus it's common to give an answer to duolingo which actually is a correct translation, but which is marked as wrong because it doesn't match their single ground truth translation. Often times, the best translation depends on context, so the user spends a lot of time learning Duolingo's biases and preferences.
A further issue is that there are many ways to translate a sentence which are understandable but don't sound natural. Language learners will almost always produce these types of sentences before they learn how to produce natural sentences. Some common examples: Chinese people learning English will often underuse articles ("a", "the") or use them incorrectly. In the Chinese film "The Wandering Earth", one sentence translates to "Jupiter is 90% hydrogen atoms", but the Chinese word order is more like "Percentage 90% of Jupiter is hydrogen atoms". The sentence is simple enough that it's understandable even if the word order isn't exactly right. I believe that it's likely part of the learning process for people to produce these sentences before making very high quality sentences. Duolingo either grades a translation as correct or incorrect, and thus it doesn't have a way of giving partial credit. I suspect Duolingo masks the fully severity of this issue by making the source and target sentences very simple.
3. Teaches unnatural sentences. I'm mostly going off of my personal experience here, but I've used several Chinese sentences from Duolingo with native speakers, and they've had trouble understanding them, due to the use of unnatural sentence structures. One example, a common type of Chinese sentence is structured like "I today drink coffee" or "wo jin tian he kafei". However Duolingo for some reason structured the sentences like this as "Today, I drink coffee" or "jin tian wo he kafei".
4. Duolingo almost always provides sentences which is free of any sort of context or associated imagery, which makes the sentences hard to remember. I found that this made it difficult to think of the words when they're relevant to a real-world situation, because the words are presented without the real-world context.
5. Duolingo doesn't teach any grammatical theory. In many cases, just explaining how part of the grammar works would be more efficient then having users learn by trial and error. For example, Chinese uses a different system than English talking about time than English. I still don't understand it very well, but it's clear to me that Duolingo's trial-and-error approach will lead to lots of confusion and mistakes. For example Chinese has a character "Le" that often means that an action is completed. This almost always implies the English past tense, but is conceptually rather different, and the converse doesn't hold.
6. Duolingo doesn't teach any etymology or explain how character construction works. Chinese characters can be constructed in various ways, but some include pictographic and pictophonetic. I won't go into the details of this, but it's much easier to recognize characters if you know to identify parts of the characters that may indicate the overall sound or meaning of the character. Duolingo treats characters as atomic and ignores this completely. At least letting users copy characters to the clipboard and search for them on an external site, while telling them that characters have an etymology, would be an easy area for improvement.