We're using Notion right now to manage user research, sprint kanban boards, and a wiki for a Code for Canada project. As part of the wiki side, we can embed InVision mocks, RealtimeBoards, Google Drive docs, and many other things directly into our pages, in a way that allows us to interact with (comment, edit, etc.) them. About the only thing I find missing is full-text search across pages; other than that, it's been really quite great as a central place to organize the whole project.
No idea if that will remain true as they scale up, but for now they seem to have found a way to break the "only works for a specific niche" pattern, and they're carving out a meta-niche of "teams whose work isn't easily captured by previously defined specific niches".
My theory here is that todo/work tracking apps are always doomed to end up like this. The problem is that we use the same words to talk about something that varies greatly from person to person, and even more so from organization to organization.
The upshot is that a company will start out with a strong vision, find a narrow segment of users who work perfectly in line with that vision, and make them happy. But then they want to expand, so they start adding features and options that let others groups find a way to work using the tool. Eventually either the tool stops expanding or feature creep and death-of-a-thousand-cuts changes make it an all-things-to-everyone nightmare (hi, Jira!).
Personally, I use Kanban Flow, which is very focused on a Kanban style approach. By the name it's obvious they're targeting a very specific niche. Since I happen to be in that niche, I love it. But I wouldn't recommend it to all comers, because for a lot of people it won't match their tasks or how their brain works.