I atended a seminar organized by CST, where they presented their softwares, now I have collegues that use CST and have good results, but I know other engineers that use IMST,QWED or Fidelity and they also get excellent results.
Now I listened quite carefully to CST presentation and they convinced me that they use precise geometrys like the real model they try to simulate(compared to stairbox case), but what they did is render a perfect geometry and move the aproximation to the electromagnetic simulator calculation.
So when using a staircase simulator the simulation is precise, but the geometry tries to aproximate the reality-so results are a aproximation,
CST removed the aproximation from the geometry and transfered it to the simulation itself.
The results are pretty good for a good operator, but other simulators will get you similar results, some maybe more precise. But we are engineers so we need enough precision to do our design and can leave absolute precision to pure academics.
CST for sure have for the time being the best modeller, but this is changing fast in this competitive market.