I really like Observable a lot, and have made a couple of brief, messy visualisations with it (messy in terms of code at least).

I hope at some point they can offer some sort of offline support. The focus on ease of sharing is great, and the volume and breadth of content has really helped me to get more proficient with d3. The inability to open and edit notebooks offline/locally is a real inconvenience, however.

I have a small personal site on Netlify (with Next.js) and going forward I'm looking at hosting the sort of stuff I have on Observable there, simply as it's easier to work to when offline. The quality of the community and resources is a really valuable thing, but the need to be online is a real drawback at times.