A problem with this particular approach is that chrome-trusted for example will no longer have access to the session's DBus, so it cannot show any notifications for example. And DBus, like X11, does not provide any separation right now as far as I'm aware (it has support for authentication, but in practice, `external` is used most of the time which is based on UID).
The only variant that might provide a real security benefit here is the -paranoid one, as that those not share the X11 server session.
Yes, X11 has no separation, but you can use multiple X11 isolated sessions, like i do in this config i'm explaining in this post.
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.