signals and slots seem like a way of introducing an extra layer of type safety and explicitness - you cannot accidentally pass the wrong function pointer to a handler, and it's easier to extract your wiring graph. they don't make things all that much harder either; they add a slight bit of ceremony, but it's worth it. (mostly based on my experience using qt)
At least in the Qt implementation, signals and slots make it easy to forget to connect essential signals, and to understand which signals are essential. I also feel like they tend to encourage making the interface more complex than it needs to be.
On the other hand, with function pointers or virtual functions, you can easily ensure a required callback is provided, by requiring the user to pass it in the constructor.
I don't see any difference related to "accidentally passing the wrong function". In either case you need to specify which function to call and on which object; for virtual function callbacks it's even easier and harder to fail. Type checking can also be done by the compiler in either case (even for function pointers at least in C++ it is possible to make a type-safe wrapper, see my implementation of lightweight callbacks [1]).
