I’ve really liked using Lingodeer alongside Wanikani, though I’m also using the Genki textbook and other resources to learn. There is a lot of overlap in vocabulary used between Lingodeer and Wanikani, so with a few exceptions, I haven’t had to spend much effort actually learning the new words that Lingodeer presents in each lesson. Several times, I’ve had Lingodeer teach me a word and it’s shown up in my Wanikani lessons shortly after.
Lingodeer also does not focus on tourist phrases, the app is aimed at actual learners of the language rather than those needing a way to ask where the bathroom is on their vacation (though it does teach you how to ask where the bathroom is, iirc). It just starts at the same place any grammar resource begins, with very basic sentence structure, and develops from there.
In terms of using Lingodeer as the sole grammar resource, I’d be a bit hesitant to advise that, if only because as you noted, it does use set sentences to teach and practice the grammar points rather than forcing you to create your own responses or find the answer without it being a multiple-choice problem. A textbook like Genki or resource like Bunpro will probably do better to let you practice the grammar points, though I personally haven’t cared much for how Bunpro approaches their ‘lessons’. Also, audio is integrated into the lessons and to my knowledge, there isn’t a way to do the lessons without it, so it does more or less require you to be somewhere private or have headphones on hand.
Lingodeer does have clear and concise explanations for the grammar points built into the app, though, and it does have a review capability to let you revisit learned grammar without redoing the individual lessons, though the reviews do not operate on an SRS system like Wanikani or Bunpro. It uses native audio and has a few customization options that allow you to switch how you want the Japanese displayed (kanji, kana, or romaji, or various combinations thereof) and the ability to switch one of the lesson activities to text input, so you get some practice with producing the words or using the Japanese keyboard on your device if you have one installed, though the sentences you’re writing are pulled from the sentences used in the lessons.
However, I use Lingodeer as a way to introduce myself to grammar points and become comfortable with the concepts and I think the app excels at that. I wouldn’t consider myself as having properly learned most of what Lingodeer has taught me so far (I’m 80% of the way through course 1), simply because I haven’t spent enough time learning the rules and using the grammar points to create my own sentences, but that’s why I use Genki and other resources. Lingodeer lets me take fifteen minutes, read about and get comfortable with the idea of a certain grammar point and how it’s used, then take that minimal knowledge out into the wild so that when I see it, I have a general idea of what’s going on or at least where to look for a reminder on how that grammar works. The app is very well-designed for that purpose, I think. And each lesson takes about 5 minutes to go through, so it’s easy to pull it out when you have a few spare minutes to look at it.
Overall, I’d definitely recommend Lingodeer as a way to be introduced to and become familiar with grammar or as a way to do quick reviews of learned grammar, but would recommend additional resources to really start internalizing it.