Be aware that those who think an application is impossible with HFSS (or any other tool) is likely very wrong. In this case HFSS excels with FSS analysis for both unit cell and finite array analysis in a domain that is natural for both applications, the frequency domain. Also be aware that ensuring a converged mesh is very important, and HFSS does this quite well in an automated fashion. CST may or may not, I do not know (and admit that) however HFSS is very capable in this application and quite efficient in HFSS12.1