If you install a wad of unknown origin, it could stub a system ios, or install an old system menu, just to brick your wii for the fun of it. Be sure to install good wads, you can view thier details in showmiiwads nicely. I don't really think anyone would try to brick you like that, but it can and probably has happened.
I wonder what happens to a wii if you install and older verison of boot2 via wad? Anyone know? Insta-brick?