I knew about voiced-unvoiced clusters. But I
didn't know about {sc} and {zj}. I can pronounce
both of them (even stand-alone, i.e. without a
vowel). Why would you want to exclude these
sounds?
Consonant cluster isn't a universally common thing. There are people who don't find it easy to pronounce because it isn't so common in their own mother language. Also some people find some particular consonant clusters easier than others for the same reason. With less number of such sounds Lojban phonology seems to have been arranged not to horrify/dismay as many people as possible. Indeed, it even allows an insertion of a small vowel (when speaking, not writing) between the consonant clusters for those who have difficulty in pronouncing it.
The mere fact that some people can deal with it without problem just won't help those who can't. I think Lojban is not meant to segregate/alienate people of particular linguistic backgrounds. And that is one of the differences between Lojban and, say, Esperanto.