In regards to the accuracy of certain additional effects on weaponskills and blue magic, my post from FFXIAH:
My current hypothesis is that it has something to do with the way that   certain additional effects resist. Currently I've only documented   defense down, blind, and accuracy down additional effects having the   problem, and I suspect that these status effects, possibly more, have   some issue regarding the specific code bits that govern these additional   effect's resist states; i.e. full duration, half resist, full resist.   The only wrench in the scheme is Hecatomb Wave, which has no specific   duration, however it's possible that even when randomized it can cut the   randomized duration in half on a resist.
Currently affected abilities and spells (the ones I've tested):
-Tourbillion (Add. effect: Defense down)
-Sandspin (Add. effect: Accuracy down)
-Barbed Crescent (Add. effect: Accuracy down)
-Hecatomb Wave (Add. effect: Blind)
-Tachi: Ageha (Add. effect: Defense down)
Ones recently tested by others:  
-Metatron Torment (Add. effect: Defense down)
-Onslaught (Add. effect: Accuracy down)
-Full Break (Add. effect: Accuracy, Attack, Defense, and Evasion down)
-Armor Break (Add. effect: Defense down)
-Shell Crusher (Add. effect: Defense down)
-Randgrith (Add. effect: Evasion down)
-Nightmare Scythe (Add. effect: Blind)
-Skullbreaker (Add. effect: INT down)
I suspect that similar abilities and spells would behave the same way. I   can't test anything else, nor can I file a report on the official   forums about the relic weaponskill(s) that are affected, as my   subscription lapsed a few weeks ago.
If anyone wants to test some other things, the weaponskills I'd want to look at next would be:
-Gate of Tartarus (Add. effect: Attack down)
-Blade: Metsu (Add. effect: Paralysis)
-Guillotine (Add. effect: Silence)
-Tachi: Yukikaze (Add. effect: Blind)
-Tachi: Gekko (Add. effect: Silence)
-Tachi: Kasha (Add. effect: Paralysis)
-Weapon Break (Add. effect: Attack down)
-Wasp Sting (Add. effect: Poison)
-Viper Bite (Add. effect: Poison)
-Exenterator (Add. effect: Accuracy down)
-Blade: Retsu (Add. effect: Paralysis)
-Shattersoul (Add. effect: Magic Defense down)
-Vidohunir (Add. effect: Magic Defense down)
-Garland of Bliss (Add. effect: Defense down)
-Omniscience (Add. effect: Magic Attack down)
-Seedspray (Add. effect: Defense down)
Abilities and spells that are 
not affected:
-Blade: Kamu (Add. effect: Accuracy down)
-Shijin Spiral (Add. effect: Plague)
-Blade: Yu (Add. effect: Poison)
-Shadowstitch (Add. effect: Blind)
-Head Butt (Add. effect: Stun)
-Sudden Lunge (Add. effect: Stun)
-Delta Thrust (Add. effect: Plague)
-Spiral Spin (Add. effect: Accuracy down)
-Benthic Typhoon (Add. effect: Defense and Magic Defense down)
-Bilgestorm (Add. effect: Accuracy, Attack, and Defense down)