X11 has long been known to be vulnerable to something known as the "unauthorized access attack" (CVE-1999-0526), which allows applications to take unwarranted screenshots, tap into keyboard and mouse events, etc., to gain access to usernames and passwords. Although Wayland has security measures in place to prevent these sorts of attacks, this protection is only within the realm of serving graphics to your display. An application can still, in practice, listen to other types of messages sent to the kernel through the keyboard.
In short, avoid overconfidence in your machine, as Wayland enhances your security in niche ways!