Which consecrations do people have doubts about with +Thuc? and why?
I would have doubts about the ones that are not adequately witnessed or proven. Anyone can go around claiming that he was consecrated by +Thuc. Then, subsequent consecrations performed by those men would be doubtful if they themselves had not been adequately trained so that one might have confidence that they could properly administer the (relatively complex) Rites involved.
+Gueard des Laurier, +Carmona/+Zamora, and even +Dominguez et al. are unassailably valid. +Guerard des Laurier's consecrations are also undoubtedly valid, as are +McKenna's ... so for instance +Sanborn. And +Pivarunas->+Dolan are valid. Now anyone +Dominguez consecrated, or some of those other chumps who probably had little training ... those are doubtful.