Werebears are very rare and even rumored to be nonexistant, though we know they actually exist. Read "Lycanthropic Legends of Skyrim".
That could be said of all Werebeast. They are all rare, even the most widespread, Werewolves have been called very rare, many believe them to be myth and they probably all try to stay anonymous lest they attract those who know of their existence and wish them dead.
Actually the guy was trying to see if rumors of werebears were actually substantiated which he never did, he couldn't even get anyone to really talk about lycanthropes except for one guy referencing the Silver-Hand. Even in the book On Lycanthropy from dagerfall which says werebears are the most common lycanthrope in skyrim the author failed to actually meet any. It is possible that werebears never actually existed and it was simply a legend people took to believe as fact.
I would say wolves, boars and vultures are the only lycanthropes we can truly confrim as existing.
The author of the book 'On Lycanthropy", Varnard Karessen, was correct on his accounts of the Werebeast that we know exist and if the pattern of DLC is kept then he was correct on Werebears as well. At this point I would say that he is a credible source and so I believe that the other Werebeast described in the book also exist. And in regards to your signiture, the author of Immortal blood was correct about the Vampires of Cyrodiil and I believe that the claims the book makes are verified even further by the confirmed existence of another of the book's subjects, Morvarth.