I'll preface this by saying that I'm a Wayland user (Hyprland, then KDE Plasma, and I'll be giving Cosmic a fair shot), and don't see myself returning to X and having to choose between massive screen tearing and massive input lag.

Wayland is missing many features that are required for some people or some applications. There's no way for a multi-window application to tell the compositor where to place the windows, for example to have one window snap to and follow the other. Color profiles were implemented very recently. Wayland's isolation of applications, while a significant improvement to security, has made remote input software and xdotool-like programs highly dependent on third-party interoperability solutions (specifically dbus and XDG Desktop Portal). The same isolation broke most accessibility tools like screen readers. Dockable windows, like the toolbars in QT Creator or QOwnNotes, are often difficult or impossible to dock back into the main window.

Because Wayland compositors have to implement all protocols (as opposed to deferring to the X.Org server; which is why wlroots is such a big deal) or rely on XDG Desktop Portal (which has never worked right for me), feature parity between compositors is never guaranteed, and especially problematic with GNOME dragging its heels.

Wayland is nowhere near feature parity with X11 today, and that is a legitimate prohibitive issue for many people. Wayland will never reach total feature parity with X11 in some areas, and that will always be prohibitive for some people.

But the worst (in my opinion) is the development process of the Wayland protocols. The proposal discussion threads read like the best and/or worst sitcom you've ever seen. It took them several months of back-and-forth just short of ad hominem attacks to decide how a window should set its icon. Several months for a pissing WINDOW ICON!