I merged spaced repetition with knowledge management app ( ), and I must say it was the biggest personal improvement for me in terms of organising and remembering stuff. As a product though I struggle to make it work. I assume people mostly afraid of their notes and cards stored in the cloud vs local storage, which is understandable. https://www.mindthis.io/
Seems nice. The page is so irritating on iOS that it must dominate your revenue losses though, not fear from cloud. Like portrait screen 4 colums of 2 character wide lines of texts. All video autostarts on fullscreen when gets into view, ehh. Have to ask, have you ever tried this?
Update: made it look a little bit better.