The "you can just send bytes over" bit is kind of the whole point. While I use Google's flatbuffers instead of Capn'Proto due to easier use w/ in-memory buffers

The "you can just send bytes over" bit is kind of the whole point. While I use Google's flatbuffers instead of Capn'Proto due to easier use w/ in-memory buffers they more or less have the same design idea: the underlying data is little-endian and stored in machine native types. For my use case in electronic trading, a format like this is great b/c you don't waste many CPU cycles in decoding/encoding while at the same time you can take complex message structures and easily generate code stubs to read/write them in various languages. You change something? Just regenerate the code stub. You can even guarantee backwards compatibility if needed.