A lot of it's down to a paradigm conflict. In general, if statements are fine. But they're not really considered to be object-oriented. Smalltalk technically doesn't even have an if statement.

I will say that, when I see object-oriented code that branches on the particular subclass of a value at run time, it's often one of the earlier sign that the code is getting messy. Subclassing is supposed to be used for a "don't ask, tell" style of programming. A lot of the problems with OOP that people like to complain about aren't really problems with object-oriented programming, per se; they're problems that crop up when object-oriented and procedural programming are mixed in an uncontrolled manner.