'It wasn't what I expected' vs 'It wasn't what I expected it to be'
I agree with your understanding of these phrases.
However, here are some other phrases that you could use that would be more casual/relaxed:
Scenario #1: "You could say "That restaurant didn't live up the the hype." or "I don't know what everyone was talking about, that restaurant was awful."
Scenario #2: you could say "I'm not sure what happened, but that restaurant isn't as good as it used to be."