Why is iostream::eof inside a loop condition (i.e. `while (!stream.eof())`) considered wrong?

I just found a comment in this answer saying that using iostream::eof in a loop condition is "almost certainly wrong". I generally use something like while(cin>>n) - which I guess implicitly checks for EOF.

Why is checking for eof explicitly using while (!cin.eof()) wrong?

How is it different from using scanf("...",...)!=EOF in C (which I often use with no problems)?