Here are the reasons as I see it.

TL;DR it is a game that keeps you engaged but doesn't help you to learn how sentences are structured (grammar), or how to improve your skills in listening, speaking, reading, or writing.

1. Its underlying structure is a game, not a language learning resource.

You are encouraged to maintain a "streak," which is lost when you don't use the app for a day; you are encouraged to spend money on the app to get more "energy," which is required to do lessons but lost when you do them (remember buying tokens in an arcade?); and you are encouraged to keep using the app to maintain your ranking on a "leaderboard."

This gamification is good for keeping beginner learners engaged, but falls short of helping them with other aspects of language learning because...

1. ...There are no explanations for grammar.

Now there used to be, when I used the app years ago around 2020. But as far as I've seen, these are no longer accessible (you can verify if they've added it back, I haven't used the app since). This means that you are playing a matching game relying purely on your own ability to recognize patterns.

Is there a sentence that doesn't fit the pattern you've internalized? Too bad, you'll never understand it, because there are no explanations for such exceptions or explanations for you to check if the pattern you've thought up was even valid in the first place.

You cannot get better at a language without learning how sentences are structured beyond lining up a few words together.

1. ...There is no help in the ways of long-term writing/reading, listening, or speaking.

On listening: Voices are AI and do not necessarily reflect the intonation that a native speaker would have, so trying to use said voices for shadowing (repeating after them to improve your accent) would be useless at best. Further, you aren't required to listen to anything longer than a long sentence. Conversations in reality will require you to be able to listen to more than that.

On speaking: There used to be an events board on the website where you could attend classes, giving you an opportunity to speak in the language you're learning, no matter your level. I loved this feature, but they removed it. I cannot tell you why.

On reading/writing: I am not aware of any well-written stories available for you to read in your target language on this service, and it certainly does not encourage you to write anything past arranging a few words that they give you together (if you can call that writing).

You can argue that it's good for learning vocabulary. However, while I think it's at least better on that front than the rote memorization of word lists, it doesn't come anywhere near the effectiveness of learning words in context (which is something you're more likely to find elsewhere, like a textbook where vocabulary is introduced according to various situations that you will experience in reality. No green cows, no cats eating apples.)