The assasin specific super katar feels weak. The weapon doesn't really grant anything on top of the +4 enchantment, keen, an additional 1 magic damage and a fairly weak massive criticals feature on top of a really mediocre crit range.
I am by no means a powergamer, and plan on taking the weapon through exotic weapon focus and UMD purely because katars look so cool. I don't believe we should expect the dev team to make sure that every single +4 weapon is perfectly balanced against all others. However, this weapon still stands out as almost unjustifiaby weak due to the shortcomings of the base weapon and the mediocrity of its added bonuses. Most top tier weapons either do great damage, provide fun/interesting abilities or grant bonuses to skills. The final blow does none of these things.