Have SEMS any advantages over FreeSWITCH for topology hiding?
More narrow and specialised use-cases, so you don't have to deal with stripping it of PBX and miscellaneous application features. It's more of a dedicated B2BUA (with its sbc module), in this case.
I tried to use SEMS but I've got error, which was reported about 2 years ago without any decision - http://lists.iptel.org/pipermail/sems/2014-August/004451.html