There is a quote in WoR, and I believe it is an excerpt from WoR in WoR (whoa), that talks about someone who could not join the Lightweavers because he was incapable of knowing himself to the degree that the Cryptics require. (I listen on audio, so I don't have the direct quote).
It seems to me that admitting truths about oneself, to oneself may be as hard, if not harder, than speaking other oaths. I mean, clearly, it is difficult for Shallan, and speaking from Earthly human experience, it is certainly a difficult thing to do.