Sure, you theoretically can migrate your notes from, say OneNote to Evernote (and vice-versa), but doing so is a great experience in frankensteining. I'm not even very confident there isn't outright data loss in some places, and with a high volume of notes, it's not feasible to check manually. Even with no loss, lots of stuff look ugly and/or do not work anymore, even if there is an equivalent on the target platform.
Take a look at Bear on Mac & iOS, and DayOne (my favorite but lacking full markdown support). Also Dropbox paper is interesting and live previews markdown and has nice table support
As a programmer, most of the notes, I take, have code snippets. With markdown notes, I get nice code highlighting in my notes.
Also a lot of times HTML/RTF notes end up having hidden invisible characters. So it messes up commands. That's why I prefer plain text notes over rich formatted notes.
If there isn't an editor to hand it is easy enough to read as plain text.
I had high expectations when Gus Mueller sold the app to plausible, but they haven't invested enough on it since 2013.