Nom is good for protocols where you need to know the schema in advance. E.g. a binary key-value store, where the binary in the key and value has to be deserialised into some special rules.

Nom can also handle bit-level parsers very easily. It's easier to use the parsers in nom::bits than remembering all the bit-shifting tricks I haven't used in five years.

But I'm not an expert on Serde, I suspect it can do bit-level too. I've never written my own Serde library, I just use crates like serde-json.