Google's rendition of IMAP isn't following any standard but their own. Fastmail uses Cyrus IMAP server, which most certainly follows standards.
I've been a Fastmail user for almost 10 years. Never an issue that was not solved in very short order and most professionally. Fastmail offer a modern product with old world service and charm. A win-win.
We use either Postbox, Thunderbird or Apple Mail and all of them should work with Gmail like every most common mail client. So the point about IMAP doesn't have a practical impact for us. Am I missing something? Good to know about their strong reliability though
I've been a Fastmail user for almost 10 years. Never an issue that was not solved in very short order and most professionally. Fastmail offer a modern product with old world service and charm. A win-win.