My memory is hazy on it, _but_ from memory, there were some issues around IDs and persistence which don't occur on other servers, as well as the auth being a bit funky. (Gmail also uses its own extension to the protocol for IDs instead of using eg OBJECTID.)

To be clear, I have no opinion on IMAP or what things _should_ do; I certainly was not using IMAP software in the 1990s :) However, trying to implement a client that works across providers does mean trying to operate per the standards, which was a struggle and why I started documenting it :)