Lessons in app are different than on the site?
For instance, in French for English speakers, the section "places" has 11 lessons on the site, but only 10 in the app. I have seen others as well, where I got words I already learnt on the site, but which came up as new words in the next lesson in the app. For people who are using both the app and the site (like me), this is pretty annoying, as I now think I might have missed words while switching between the two. I'm thinking about just using the site on my tablet, just to make sure I'm not missing anything.
I just use whatever's convenient, be it the website or the app.
The entire Duo course teaches less than 2000 words, and I plan on learning many more than that. I don't care if I miss a word here or gain a word there...I just enjoy the ability to learn for free no matter where I am.