good question, comparison beyween projects is a good investigation method. they are really similar projects. flexx has more modularity and depends from powerful libraries like Tornado. Remi has its own webserver and has zero dependencies, not necessarily a good choice but however a notable difference.
How does this compare to Dash by plotly and also Jupyter Dashboards?
At first glance the use cases seem different (applications vs dashboards) but they both provide widgets and GUI elements. Are there things that would be much simpler/quicker in one vs the other?
correct, they seems different but you can achieve "same results" with both. dash is really powerful for look n feel and for plots. but I see it a bit confusional for callbacks and object oriented development. I never used it, so maybe I'm not the right person to ask for a comparison.
Seems great! I used https://github.com/miyakogi/wdom a few times but it wasn't satisfying because you have to get your hands dirty with js when you want to customize things (not in js libs but with the widgets).
How does remi compare to https://github.com/flexxui/flexx?
For example, flexx can translate parts of the app to JS to have a more reactive experience.