I really, REALLY should make a guide about that >_>
Weapons that hit more than 3 times in a combo (daggers, double sabers, paired sowrds, mechguns) or weapons that hit more than one target (slicers, partisans, swords, shots) have reduced specials, some get a 1/2 reduction, some a 1/3.
So, for instance, if Gush steals 90 HP with sabers, it will steal 45 with daggers or 30 with mechguns.
Some rare weapons have a major advantage by not getting the reduction, like Demolition Comet, or Guld Milla, etc.
The only special attacks that cannot be reduced are Charge, Spirit and Berserk (and all non-regular specials, like Lavis Blade, Berdysh, Soul Banish, etc.)
That's the gist of it, I'll make a guide, someday, or someone else will, whatever