I was reading quite an interesting piece on the historical claims of a controversial, but very well-known art and the author wrote that even though the lineage has not been proven beyond the 20th century, this does not consitute a problem because ninjutsu is a "Kuden" art.
Does this sound reasonable?a vast number of Ryuha in Japan (across martial arts, flower arranging, tea pouring, etc.) are considered "Kuden." I'm sure the thousands of instructors of Kuden arts would be quite unhappy to hear that their arts can be completely dismissed merely because a group of 100 people who follow a different lineage vehicle say so. BOTH "Kuden" arts and "Koryu" arts are valid. Period.
My own point of view is that even if teachings were kuden, there should still be some documentation or proof that the school itself (tea, budo, flower arranging etc.) existed.