The real answer about +Thuc was that his episcopal lineage is both good and bad. The schismatic sect in Spain is due to +Thuc's actions...Even if we give him the benefit of the doubt, his lineage is tainted. Some go to the extreme and say his ENTIRE lineage is bad, which is a horrible conclusion.
History says that +Thuc consecrated a LOT of bishops. In the wild west days we're in, some turned out good and some turned out bad. Should +ABL/+W be blamed for the actions of Fr Pfeiffer? Of course not. There are MANY good bishops who exist due to +Thuc. There are many fruits of his actions, even if some bad apples.
The +Carmona/+Zamora and +Guerard des Lauriers lines of +Thuc consecrations are unimpeachable. Most sede bishops and clergy trace their Orders to these lines.
Careful studies of lines from consecrations by bishops other than Thuc will also turn up good and valid as well as doubtful lines.
Sacramental theology teaches that one always approaches a sacrament with the presumption of validity until one can morally determine a problem with matter, form (which also determines intention), minister, or recipent. If a problem does arise with moral certainty, then the sacrament is treated as doubtful.
No one may consider a sacrament to be invalid until the Magisterium has ruled it as such, and the Magisterium is currently in suspension. Therefore, no questions of sacramental validity arising post-Vatican II can be considered definitively invalid until the Magisterium is restored and rules such. This principle even applies to the Novus Order sacraments -- where form, matter, minister, or recipent has been altered, these must treated as doubtful until a restored Magisterium can rule. No one currently has the authority to declare them invalid.