It is something I didn't really care that much when using X11 or that I tolerated because the alternatives (win and macos) were worse for me from an usability standpoint but once you are used to wayland it is hard to go back to x11 and its occasional tearing.
I guess it is like a lot of things ignorance is bliss. Appart from my smartphone I have never used a screen with a high dpi display so I am fine with 1080p on my computers but it may change the day I switch at least one screen for a 4K display.
performance: holy shit it's so buttery smooth, especially if you're on an intel or amd graphics stack. under x11, it felt like it was impossible to actually eliminate tearing everywhere. under wayland (my experience is limited to sway, gnome, and most recently kde) tearing just ceases to be a thing.
Any process you run can already access the memory and files of everything else as the same user.
Wayland's security properties are over-sold and practically irrelevant, and it doesn't help that it breaks a ton of functionality that people need in the name of this "security", like screen readers.
*When the applications themselves aren't properly sandboxed. If they are with for example firejail, flatpak or snap. Then x11 is a gaping hole like having proper doors in a house but broken windows.
It's not the job of the compositor to sandbox applications.