I know at least one Ford ECU (1996 Ford Escort ZETEC) which has a limp-mode backup controller if the main ECU processor dies, so no, at least some ECUs have redundancies inside.
Oh, I'm sure a lot of engine ECUs have redundancies, probably most at this point. I was speaking toward the Magnetti Marelli Digiplex ECUs from the 1980's specifically. They have no redundancy.