Radiation tends to be more serious as frequencies increase. When circuit boards were running at speeds of tens of megahertz, finding a trace or any other structure that was as long as a quarter-wavelength was rare. At multiple gigahertz, all sorts of structures can become efficient radiators. At tens of megahertz, signals traveled down wires. At microwave frequencies, signals can travel quite well down the gap formed by the edges of two adjacent planes. What we used to think of as a plane-split, the microwave people call a slot-line. At the right length, it can make a nice antenna. A signal can travel around the outside edge of an isolated plane. If that periphery is the right length, it can make another nice antenna. The microwave guys call it a patch antenna. The point here is that structures that were of little interest at lower frequencies can become very interesting and perhaps troublesome at microwave frequencies. These transmission lines that SPICE doesn't know about can provide many of the same services, crosstalk and such, that good old wires used to provide.

Circuit solvers such as SPICE do not know anything about radiation. Any time it is necessary to model radiation, expect to be using a 3D full-wave field solver.

Of course testing real boards is an important part of studying emissions. It often is not practical to build a full 3D model of a circuit board with all the components, heat-sinks, cables, chassis, and so on that...