I’m only going by my personal experience with this, but I switched all my learning over to the android app I think around April last year and shortly after switched back to web only learning.
What I noticed is that besides being given letters which makes it easier (though the option exists to show the full keyboard), and points being off, there is a difference in the “punishment” (more lenient) after getting something wrong.
I also had a strong feeling that the timing schedule of when words would be reviewed was different.
Again, I didn’t sit down and really try to test for differences/bugs, so I don’t know if the reviewing schedule was messed up normally, or only after getting something wrong. I do remember clearly though that multiple times I missed a word, was correct on other words from the same learning level, and the word I got incorrect was moved to be reviewed in something like 5 days where the correct words were a shorter time period. I suddenly started noticing that my retention was consistently ~75% whereas before I would rarely miss answers. I got kind of discouraged until I realized that all my problem words coincided with the words I learned on the app.
Additional things I found is that doing speed review actually affects the schedule of words tested in it. This is crazy I think. Speed review is good as fun and to maybe test your speed at reviewing levels, but there are just way too many variables that could allow you to cheat or that could give you hints but this affects the actual learning schedule.
This all really stood out to me, because I started using the site in 2012 and the thing that really comforted me was the fact that if you put the time in, and water your plants/review within a couple of hours of when they are ready to be reviewed, you really couldn’t go wrong. They had actually, at least for me, figured out the exact timescale of when planted seeds would get hazy in my memory, prompt review right then, and that would then solidify the memory. It was (and still is on the website), like cheating while learning but without cheating yourself. Instead of kind of winging when new things should be learned and old ones reviewed, it just made everything super efficient. The fact that this, the actual genius of the website as a nearly foolproof way to master how the brain memorized vocab, was less a focus in the app transition than was artwork for handfuls of planets that scroll by nonstop when shifting through levels was a really big red flag.
The thing is that since the review method is easier in the app, it’s not standing out to people that there is less efficiency. You can just mindlessly tap while colors are exploding around you. It’s more of a game similar to the speed review. That should totally be something that people can use for hours at a time like a very productive game, but so far the raw learning in the app does not equal the website. I am just keeping my fingers crossed that the review algorithm isn’t altered on the website and I can get through my courses before anything drastic happens.