> Although I see their point about non-REST, I think there's still a case for having a REST flavour available even if it may not be the most efficient.

I was pretty disappointed when I found out how ugly are JMAP requests. I'd also like to see REST variant as messages and their parts should be easily mappable to resources. Too bad SMTP was not also included in JMAP. ActivityPub comes really close to SMTP over HTTP/JSON in my opinion.