There is difference with free software and proprietary software. NextCloud instance is free software and holds encrypted information. Maybe there could be ways to intercept password by the ISP.
This application is proprietary software, and you don't know nothing what is going on in background. Just as majority of Service as a Software Substitute, or SaaSS steal people's data.
Mamma mia!
Why not pay US $5 to Digital Ocean and run your own NextCloud and you have all the notes in your own server. Encrypted.