It uses the same method, but the focus of openEMS is more in RF devices while Meep is more focused on optics. One important difference is that openEMS supports graded (inhomogeneous) meshes. You need this since for RF devices some features are much much smaller than the wavelength, e.g. feeding lines, and you do not want (or need) a dense mesh everywhere in those cases. Furthermore openEMS has a small GUI to display the defined structure and mesh to inspect if your setup is ok.