The big challenge with the approach not touched on in the post is version skew. During a deploy you'll have some new clients talk to old servers and some old clients talk to new servers. The ViewModel is a minimal representation of the data and you can constrain it with backwards compatibility guarantees (ex. Protos or Thrift), while the UI component JSON and their associated JS must be compatible with the running client.

Vercel fixes this for a fee: https://vercel.com/docs/skew-protection

I do wonder how many people will use the new React features and then have short outages during deploys like the FOUC of the past. Even their Pro plan has only 12 hours of protection so if you leave a tab open for 24 hours and then click a button it might hit a server where the server components and functions are incompatible.