The problem Notion solves arrives around source number eighty, when your reading notes stop being findable and you start re-reading papers you already summarised.
As a research database it is straightforwardly good: structured properties per source, tags by theme, and AI that can query across your own notes rather than the open web. Asking which of your sources discussed a particular method, and getting an answer grounded in notes you wrote, is the useful capability.
The AI is average in isolation. The value is that it operates on your material. If your notes already live somewhere else, this is not worth migrating for.