It's actually designed for this use case around spaced repetition and it's designed to be a knowledge manager AROUND spaced repetition tools.
I should have cloud sync support in the next release (1-2 weeks) which will allow you to keep your data in the cloud and sync it between machines. Initially it will just support Firebase but I have plans to support other cloud providers via plugins.
I'd also like to support end to end encryption so that you don't have to worry about people reading your data.
There's a huge Hacker News about Polar here:
https://news.ycombinator.com/item?id=18219960
It also supports PDFs, text and area highlights, comments, flashcards and sync with Anki.
The initial response after our release has been amazing. The user base is really engaged with thousands of monthly active users and contributors.
Anyway. Take it for a spin. It's free and Open Source.