I think it depends on your proficiency. I had typical classes for German and Spanish in middle and high school and found Duolingo fine for learning vocab (though I haven’t used it in a few years so ymmv).

Switched to Busuu (premium) two years ago and I definitely felt like I was progressing faster than with duo. I especially like the review feature for both vocab and grammar. Grammar in particular is taught directly whereas (at least when I was using it) duolingo meant for you to infer grammar via usage which limited my understanding.

Busuu also has a feature (one lesson in each module) where you can practice speaking or writing and a native speaker will give you feedback. I personally didn’t find this feature super useful and sometimes skipped the lessons using them.

They’ve just introduced an AI conversations feature for a few languages (english to german french and spanish I think) but I haven’t figured out how to update to that version yet. Potentially useful since I feel awkward about messing up in front of real people even if they’re on the internet.

Before subscribing to premium you could only sort of use the app because it would get stuck on a “do you want to upgrade” screen without a way to x out and you would need to close and open the app. Not sure if they fixed it or meant it to be that way for free users but it was rather annoying.