It's not open source. If migrate from Evernote, at least it's better to migrate to FOSS. Sadly, most FOSS solutions lack mobile app support.
Furthermore, FOSS is orthogonal to support and usability. It's a meaningless property on its own.
I like open source a lot, but for my data I much prefer interoperable. I've watched too many projects and companies die and take my stuff with them.