Having covered a list of items that are not particularly well handled by typical implementations of SPICE, you now get the other side. Don't get the idea that nothing works in circuit solvers at microwave frequencies. A lot does work. As pointed out above, there are things that cannot be handled without the aid of field solvers, but the idea is to characterize features with the aid of the field solver, translate that characterization into lumped models that SPICE can deal with, and then do the signal integrity work with the SPICE tool. Any time such a translation is made, there is a frequency range wherein those models are valid. Any time such a model is generated, it should also have the frequency range of applicability specified.

Again, I want to emphasize that there are numerous tools circuit solvers which, at heart, are versions of SPICE. The one critical property that such a tool absolutely needs if it is to be useful at microwave frequencies, is the ability to work accurately with transmission lines that have frequency-dependent loss. Without this capability, it is very difficult to get useful information out of a simulation. That is not to say that without such capability you are not going to be able to work with microwave frequencies; rather, without that capability, reconcile yourself to working with some tool other than SPICE.