Every few years I used to migrate my notes from one app to another, and every migration lost something: a tag here, an attachment there, once an entire notebook. Plain text files do not have this problem. A Markdown file written in 2016 opens exactly the same today.
The setup is unremarkable: one folder, one file per topic, filenames that sort well (2026-09-14-plain-text-notes.md), and full-text search with whatever tool is at hand. Sync is a solved problem; pick anything that copies files faithfully.
The real benefit is psychological. When the tool is boring, the writing has to carry the weight — and it turns out most of my notes were only ever for me, six months later, wondering what past-me was thinking.