Highlights include Sliding Sync (instant login/launch/sync), Native OIDC (industry-standard authentication), Native Group VoIP (end-to-end encrypted large-scale voice & video conferencing) and Faster Joins (lazy-loading room state when your server joins a room).
True, but the problems you mention really are a thing of the past (I’d like to get your feedback after using latest versions of maintained clients), and message delivery across multiple devices and networks does, in my experience, work much more seamlessly and reliably than on Matrix (I mean, there must be a reason why google is using XMPP behind the hood to push every single notification on every single Android device on earth, right?).
And those are client/polish issues anyway, delivering messages at enormous scale has been a solved problem in XMPP for two decades, the protocol hasn’t changed, essentially, but Matrix still hasn’t found a solution to their self-inflicted problem: https://telegra.ph/why-not-matrix-08-07