Author here. Don't quote me on this, but I think something like 90% of Duolingo users are mobile. Mobile is more "fun" and good for casual users who want to do 5 minutes everyday to feel like they're making progress. I imagine Duolingo's business model leads to it heavily promoting the mobile platform, but thankfully the desktop version is quite good too.

I didn't like the Anki UI at first but I got used to it quickly. My Anki pace of 50 new cards a day was very aggressive, but this was largely possible due to the massive number of cognates between English and French. I imagine the same will hold for you in Spanish, though perhaps to a slightly lesser extent. I think Anki is wonderful for acquiring vocab though, so keep at it and you should do quite well! Also consider making your own Anki deck or using sentences instead of words. While this 5000 word deck was good enough to get to B2 for me, those methods are very likely more effective for long-term retention of more obscure words, although of course they are far more time-consuming.