> "the only reason I still keep a Windows VM."

Another key point: its dependencies on native but very outdated Windows libraries.

No plugins, no way to modify or improve anything. Fully closed source. Feature request? Wait months until your email shows up in Dr. Wozniak's queue for incremental-reading of his emails. And one year until maybe the next version of SuperMemo. Or write the request on his wiki, to be debated by community members. It's so impractical.

These are all fair points, and yet SuperMemo meets my needs better than Anki.

Which is a thing I experience quite often. I love the idea of open source, but often the proprietary products are „better“ for what I need them.