The position of the Wayland project is that security isolation requires sandboxing. Otherwise, applications can do whatever you can do, including changing all your personal config files and scripts.

The security considerations in Wayland are primarily aimed at not being a weak link undermining sandboxing efforts, which is the case with X11.

Without sandboxing, writing to e.g. ~/.profile would in most cases be enough for a malicious application to take over the machine. Equivalents apply to all platforms.