My first impression is that the API seems a little convoluted, but that might just be me. In an ideal world, I'd love to just build components (similar to React components) on the server side, and let the framework intelligently handle synchronizing state over the network (Phoenix LiveView seems closer to that), but this feels like it involves a lot more framework-specific logic (and markup) with special cases - stuff that's likely to change as the framework evolves, so I'm not sure how I feel about it.
I guess it makes sense in terms of making this easier to add to an existing rails app incrementally (since it's more-or-less "opt-in" for each model, view, and controller), but if I'm building a new project with this and want it on everything, it feels like it will necessitate a lot of code duplication. Either that, or I use it judiciously only when absolutely needed (and use traditional rails behavior for everything else), but that feels like a mess of two very different approaches bundled together in one codebase.