Mostly that it's a relic of the late 90s/early 2000s and hasn't really aged beyond that. Group chats are clunky, it's based almost entirely in XML which has led to clientside parsing vulnerabilities in the past, it's not exactly mobile friendly with its reliance of an always on connection to servers over Matrix using push notifications, end to end encryption is a third party plugin that may be absent in some clients, to name a few. There's a reason it, along with the other proprietary messenger apps, fell out of favor and why some of the larger XMPP servers have begun to shut down over the years.