Nothing in this world is guaranteed. That doesn't mean it's uniformly random either. LLMs can still do something unexpected if you give them clear instructions, but that doesn't mean it'll be arbitrary and unpredictable in scope. The same way C/C++ undefined behavior technically means program can give you nasal demons, but in reality it won't do anything unusual (like format your C:/ drive) unless someone purposefully coded it to do that.
This is all going to flash through your mind when your car mysteriously doesn't turn left. I would prefer to think of machines as things with defined outputs and failure is failure, more than as fluffy little kittens who might do the wrong thing, if the consequences are going to fall on someone who doesn't deserve it.