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It is not safe(long term wise) for me as long as it relies on a closed source app that develops a probably dead in a few years company.

It is open source. The code is here https://github.com/standardnotes

Plus you can even self-host and the mobile apps support this out of the box.

In addition to it being open source as mentioned, the server is open source too and you can run your own self-hosted instance: https://standardnotes.org/help/47/can-i-self-host-standard-n...