Some cool things I notice about unique items. So far I have tried Youmu's swords, Shanghai, Inu Sakuya, Hisou Sword, and Mini Hakkero.
My comments are that they are all better than the normal non-unique items you can get. Their usability makes them better, even if their stats aren't as impressive. The Hisou Sword is kind of hard to use, so I am still getting used to it. On note, I don't have a deadly modifier anymore, so I cannot see how it modifies these weapons.
Youmu's sword - As already mentioned, full invincibility and life steal on ultimate makes this weapon great. AI does not use this weapon that effectively, so it is better if you use it and have the AI use a high damage range weapon, like the Mini Hakkero.
Shanghai - Auto aim as secondary makes targetting very easy. The damage is lower than Inu Sakuya and regular dolls, but it is worth the damage nerf for the targetting.
Inu Sakuya - Better range, better primary, makes this weapon a bit better than dolls. However, dolls are still better in some stages, because knives do not go through walls, unlike lasers.
Hisou Sword - This one, I still don't get how it works. It seems like you auto parry during your second and third attack patterns or maybe pattern 2 to 4, I am unsure. You always seem to do 1 damage on the second and third attack. The first attack has random damage, but it may not be random... The secondary ability for this weapon can be huge damage. It seems like damage is related to distance. At 2500ish attack power, the distance tested was in the main map, on the south side to the north side and I ran to follow the keystone. With that much distance covered, it does a really high amount, about 90,000 damage.
Mini-Hakkero - Improved primary and movable Master Spark makes this weapon much better than the Hakkeros.