The obvious problem with steer-by-wire is that in the traditional design, it's not uncommon to lose power assist but not the mechanical connection to the wheels, so you can still steer the car. To completely lose steering control you'd need significant mechanical damage.

If the whole thing goes through the computer then there are lots of new ways to fail. Steering wheel position sensor goes bad on the highway? Computer gets bad data. Control wires get disconnected or damaged? No data. Completely unrelated wires get shorted and fry the computer? No steering. Anything pops the wrong fuse? No power, no computer or steering motors.

Some of those can be mitigated with redundancy but you're still vulnerable to common causes. You have three position sensors and someone dumps their beverage down the steering column, are there any left and do you have any good way to determine which one(s)? The vehicle took some minor damage allowing water to get somewhere it's not intended to, any way to guarantee you're not about to lose both sides of a redundant electrical system the next time it goes through a puddle infused with conductive road salt?