To me, the biggest problem of this kind of tool is that if you use it to its fullest potential, it necessarily becomes a crucial part of your daily life, something that will cause very serious problems if you lose it.
And that means anything proprietary or closed SaaS is an absolute no go. My Mind's promise to respect privacy and stay independent of outside influence is nice, but worthless. They can change their mind at any time - or get bought up, or go bankrupt. Being a paid service might reduce the likelihood, but does not eliminate it, especially not over a timespan of decades.
So at an absolute minimum, such a tool needs to be able and willing to export all of my data in an easily processible format. This is far more important than shiny features.
My current solution is a self hosted TiddlyWiki. It may not have all the shiny features, but it is mine, and always will be, even 50 years from now. The worst case scenario is that it stops working on new browsers and nobody maintains it anymore - but I still have the data. Easily processible text files are actually its internal storage format.
this appears to be source available not open source