As much as it seems to be recommended against (by... authors of serialistion protocols?) I am a strong believer in just using simple structures (and unions if necessary) directly --- all these serialisation abstractions appear to have been invented at a time when machines varied far more widely in their characteristics such as endianness, alignment, word size, integer representation, and even byte size. Now that your platform is almost certainly going to be x86 or ARM, it makes little sense to add a layer of (sometimes substantial) complexity in essentially attempting to accommodate flexibility that won't be needed. I can see the necessity if e.g. you need to communicate with a 36-bit 1's complement mainframe, but otherwise it's just bloat.

Along the same sentiment, I'm not a fan of APIs using JSON and/or XML or some other overly-flexible textual encoding. Simple binary encodings, TLV-ish if necessary, are the best.

I was never really convinced by the "human readable" argument for textual encodings either --- you just need to get used to it, then you can read and write the bytes in a hexdump as easily as you can English. In fact I'd prefer working with hexdumps to XML. But unfortunately there's now a whole generation of developers who can't even count to 2 in binary and don't know what a hex editor is...