The challenge is those labels aren't standards-based. Gmail has all sorts of weird issues when you interact with it over IMAP. Even Google's own Takeout will end up making multiple copies of your messages, one per label, as it converts to folders. For a while I still had Gmail but used platforms Google didn't support with first-party apps, and I had a lot of pain points. (Gmail also likes to "archive" items [or remove that particular label] that you tell Gmail to "delete" via IMAP. It's annoying.)
FastMail does an excellent job hanging on real standards, and so supporting these Gmail-specific features requires that the standards themselves, support such things.
In the spec, they supercede IMAP folders with “keywords”, which are comparable to labels in Gmail.
“Users may add arbitrary keywords to an email. For compatibility with IMAP, a keyword is a (case-sensitive) string of 1–255 characters in the ASCII subset %x21–%x7e (excludes control chars and space), and MUST NOT include any of these characters: ( ) { ] % * " \”
Indeed, and as the case is "The data model is backwards compatible with both IMAP folders and gmail-style labels".
Even so, unless you have a wide multiplatform selection of JMAP-based apps which work properly with labels, you are better off with folders.
EDIT/Am at rate limit: I survived migrating from labels back to folders, and probably wouldn't start using multiple labels again, should they be available, until a point where it's a very common, well-supported standard behavior. No more proprietary nonsense for me.
It really is coming in JMAP! Come along to IETF in London to have a play, or I'll be pushing out links to updated JMAP Proxy in the next couple of weeks and you can play with it there :)
