JSON can't carry binary data because it must be UTF-8. Glancing over the spec, msgpack commits the complementary sin, it can't carry UTF-8 data because it can only carry binary data. Yes, it is possible to stuff some UTF-8 bytes into a binary, but you still semantically have a binary, which could be anything. You must bring additional external information to the party to know what that binary hunk actually is.
In theory this may sound like no big deal; in practice I've observed in similar cases it's a disaster. Average developers routinely muck this up (and that's just me being conservative, above-average ones can choke on this problem too). msgpack really ought to have a dedicated string type, and either declare that this string is always a particular encoding, or give a way to declare what encoding the string is in. (The second is more flexible and arguably more correct, but in something like this where there's going to be dozens of libraries trying to implement it, it is virtually guaranteed that a number of them will muck the variable encoding support up badly, so in practice I'd go with mandatory UTF-8 too.)
In theory this may sound like no big deal; in practice I've observed in similar cases it's a disaster. Average developers routinely muck this up (and that's just me being conservative, above-average ones can choke on this problem too). msgpack really ought to have a dedicated string type, and either declare that this string is always a particular encoding, or give a way to declare what encoding the string is in. (The second is more flexible and arguably more correct, but in something like this where there's going to be dozens of libraries trying to implement it, it is virtually guaranteed that a number of them will muck the variable encoding support up badly, so in practice I'd go with mandatory UTF-8 too.)