Outside of that, it's tough to compete with JSON in the "human readable unschematized serialization format" market, especially targetting JavaScript:
Use in the browser requires some degree of bundle size increase, since the parser code needs to be loaded before your format can be used. WebAssembly libraries are usually quite large compared to a pure-JS implementation. According to [bundlejs](https://bundlejs.com/?q=%40duper-js%2Fwasm&treeshake=%5B*%5D), @duper-js/wasm weighs in at about 488 kB uncompressed, 159 kB gzip.
Use in any JavaScript runtime means you're competing against the runtime's native `JSON.parse` and `JSON.stringify`. In v8, these are very quick and have runtime-level tricks to go faster, for example see [v8's recent post on making JSON.stringify 2x faster](https://v8.dev/blog/json-stringify) when serializing plain objects with no funny business .toJSON methods, replacer, or indent formatting.
Besides those points, my major complaint about JSON is how expensive it is to encode binary data for transmission; in JSON I usually do base64, with your format it's transformed to escape characters that are less efficient than base64, right? \xNN is base16 with 2 extra bytes wasted on the \ and x, or \uNNNN which is base 10 with 2 extra bytes. Is there a way you can fit binary with no expensive encode/decode step into the format?
So, for me this seems suitable as a config file format: there you get good benefit from comments, identifiers, easier string authoring. Not sure I need the binary raw string thingy in config files that much, but I guess it doesn't hurt.
The config file transpiration to JSON idea is quite interesting. It's pretty similar to how I'm already defining the TextMate grammar used by the website's syntax highlighter, so I'll certainly try to incorporate that into the tooling.
Making them part of the language would increase the complexity of parsers - how would you validate that a date is actually valid? It's doable (YAML and TOML do it, after all) but requires extra steps.