My pet peeve with vCard is the lack of an encoding and the resulting mess with various implementations. JMAP fixes this by enforcing utf8. You have my vote. :)
To be fair the newer versions of vcard also specify this, and I haven't had a single encoding issue with even the older versions. Every server just seems to send UTF-8, no questions asked.
So far all my phones had a different way to encode Unicode strings. When I wanted to merge the data, I tried a bunch of Python libraries and all failed differently. In the end I wrote my own one-off script.