I don't know if Forbidden chests are worth it, but the 3 times i opend them i just got items that could drop from the Canthan Chests....(to be more precise a longbow, Omnious Eidolon, Mammoth axe)
The drops are pretty good IF you get a gold drop. They can drop forbidden items which would be nice if you got a max gold one but I haven't seen 1 yet. Opened about 15 chest so far and only 2 max weps...both of which are Oni Daggers. Those seem to drop a lot from there. Most of the drops are purple it seems though, so I'm not going back to those anymore...to difficult to get the keys.
I would just like to note that the chests ONLY occur in the mission - NOT the explorable area. Neither are they particualarly prominent in the mission. I've had more keys drop for me as loot than I've found chests to use them on.