nullptr has type std::nullptr_t. It’s implicitly convertible to any pointer type. Thus, it’ll match std::nullptr_t or pointer types in overload resolution, but not other types such as int.
0 (aka. C’s NULL bridged over into C++) could cause ambiguity in overloaded function resolution, among other things:
f(int);
f(foo *);
(Thanks to Caleth pointing this out in the comments.)