Their imap gateway is nonstandard to say the least. The labels vs folders thing is already a mess, but it also speaks a protocol that is just different enough from regular imap that most clients (e.g. thunderbird) have a different implementation with quirks for google-imap.
Using those clients is sub-standard with google's imap implementation, and switching between them is more painful due to how each one handles googles "labels not folders" quirk