Personally I love the area and the Assassins backed by the sneak attacking black guards. Reason, we have wizards and clerics complaining about how hard it is! That in it's self shows the area is feared/a challenge.
If you think the area is hard being a wizard with Acid Sheath, Stone Skin (10 DR), PfE (stops their spamming of Doom), Imp Invis (50% conceal FTW), Mage Armor, Shield, Bulls, Endurance, Cats, Haste....all on self of course, try playing a rogue or fighter in this area. With a Rogue, sure you can spot them, but they still get sneak attacked since they group stomp you and only one won't get the sneak attack. Without the wizard buffs your down in a round unless you can manage a 40+ ac. Even then the monsters uncannily roll 20's 8 times in a row and can still bring you down. That there is Assassin's in that area which DA for a fort save of less then 20, just shows the need to group up.
DA is a one hit win. Much like Finger Of Death a wizard spell (among others), except with FoD there is not a to hit roll and with DA you need to actually finish the kill by striking the thing more. Is DA unbalanced? In comparison to Cleric and Wizard buffs, hardly. People hate sneaks since they can't be seen. Due to complaints in the past about how powerful sneaks were, the dev team has implemented changes which I feel has unbalanced the environment against sneaks now so that few bother playing them. You can't dungeon with a sneak anymore since everything has godly listen and spot skills. You wear a homing beacon and if you open that lock or remove that trap, the DC to open or remove is subtracted from you Hide and MS skill against every monster in the area. You get boot stomped.
As for Forest Fane area, hats of to the Dev team for making an area that is well rounded. Just a few tweaks to make treasure match and will be awesome!