3D Arbitary EM simulators VS 3D Planar

I have seen many posts here where people are using Full 3D EM solvers(HFSS, CST etc) for Planar Geometries(like Microstip Couplers, Antennas etc). Does anyone know any advantage of doing that?

My understanding is that planar solvers are the best for Planar geometries. Also it takes less time to learn and input your gemetry in a planar solver as suppose to full 3d. Also the simulation times are lot higher in Full 3d solvers. Does any one know of any benchmarks that compare the resuts and simulation times of 3d planar and 3d arbitary EM simulators?

Thanks

The big differences are cost and speed. Planar 3d are 1/3 the cost, and probably 2-5x faster. Also, you tend to have to "approximate" things, like via holes instead of directly analyzing them.

Full 3d are more accurate/generalized.