Wayland have some advantages but also some disadvantages, and it isn't yet really mature.
Also, wayland, on the sandbox side, doesn't do anything special. As it's just a lot simple than X and does pretty anything except copy a buffer on the screen, it just delegate all IPC and rendering to the compositor, then, in the wayland world, the security issues of X11 are moved on the compositor instead to stay on the X server, but the issue are the same and the compositor must consider them.
( anyway, for sure fix those issues in a compositor is easier and consume less resources than on X11, and then the wayland world is better from this point of view. But i don't think it's yet enough mature, and many WM/DE doesn't yet support it or support it only partially )
I know it is very immature (basically unusable in most cases), but my understanding was that it would be easier to fix these issues moving forward because of the design decisions taken in Wayland. You response seems like a tentative affirmation of that, which is encouraging to me (for whenever I can actually used it).
Wayland have some advantages but also some disadvantages, and it isn't yet really mature.
Also, wayland, on the sandbox side, doesn't do anything special. As it's just a lot simple than X and does pretty anything except copy a buffer on the screen, it just delegate all IPC and rendering to the compositor, then, in the wayland world, the security issues of X11 are moved on the compositor instead to stay on the X server, but the issue are the same and the compositor must consider them.