So what we can do is go to Bartlett's website and find evil weapons.
https://w.atwiki.jp/bartlett3/pages/313.html
Slashing/Piercing/Blunt/Ranged
Magic/Breath/Geomancy/Magic Def. Bonus.
Fire/Wind/Lightning/Light
Ice/Earth/Water/Dark
So they have magic damage -12.5%. Not ideal, but not insurmountable. They have the standard 100 magic defense bonus. Assuming Bartlett is correct this likely means they are WAR and not RDM.
We have to adjust for Sheol C. Arcana take -50% Piercing in Sheol C. So not a problem for Great Katana. The resistance ranks get increased by 4 for Sheol C. If we look here:
https://www.bg-wiki.com/ffxi/Resist#Resistance_Rank
and adjust them by 4.
Fire 150% becomes 85%
Wind 130% becomes 70%
Lightning 130% becomes 70%
Light 150% becomes 85%
Ice 100% becomes 50%
Earth 130% becomes 70%
Water 130% becomes 70%
Darkness 100% becomes 50%
These are resistance ranks not elemental damage taken. They are used to determine Skillchain damage and magic resists but crucially not elemental damage taken. The key cut off point generally is 60%. If the value is 50% or below you are guaranteed a 1/2 resist or more which magic accuracy can't overcome. (You have to change the SDT tiers (rayke etc.))
So we can infer that Ice and Darkness will get resisted. So Blade: To's magic portion and Leaden Salute will get at least a 1/2 resist.
All the Great Katana hybrid elementals are above 60% though so all should be viable. You should likely use Jinpu though because it's 2-hit and self skillchains even though fire and light are higher values.
But speaking from experience they don't do as well as we would expect. So what is going on? The answer I suspect is Shell. If you don't get to them before they buff themselves. They likely have Shell IV combined with their natural 12.5% magic damage reduction. Spamming dispelga should bring the hybrid WS back up to reasonable damage.
I should also point out that we are deducing their stats rather then measuring them and you can't take them as absolute fact. We're still guessing. Maybe there is a RDM variant of weapons that would have Magic Defense Bonus too.