Slate works well for editing text. Anything with a more complicated layout / UX gets really messy though, since with Slate, you always have to deal with the DOM / Slate's wrapper around contenteditable. And at least before Slate 0.50+, we (& the developers of Schul-Cloud independently of us) ran always in bugs, etc. This is why Edtr.io (similarly to ReactPages, fka ORY Editor) builds it own state management & layouting outside of Slate. And let Slate do what they do best, editing rich-text.
Well that could be said for literally any 3rd party library. New updates may break things, that's why you need to pin the exact version you use to prevent things breaking.
I would rather use a library which has been tested and used by many people, than reinvent the wheel on a bunch of things...