I'll vouch for the nagware. When I was studying with duolingo, it was the nag features that got me to practice every day.

A lot of apps are nagware, but I’ve never seen any as blatant and forceful as Duolingo.

They diligently A/B test their notifications, constantly looking for the variations that’ll show a higher click rate. They’ll hit you with “[Your friend’s name] will hate you forever if you don’t do Duolingo right now!” if you start slipping.

I’ve dropped it after a year or so when I realized that I wasn’t really learning any Turkish, but I was caught in some sort of corporate-designed psychological trap.