I've been doing some rummaging around JP FFXI information sources looking for information we might not have on the English side, and found some nice goodies. Most of what I found were on the Japanese FFXI wiki, but I also took info from
https://luteff11.livedoor.blog/archives/58179576.html
and
https://twitter.com/MBZ2arZNkm174sV/...94041722724352
Status Resistance and Immunobreaks
In addition to their elemental resistances, every mob in the game also has a selection of status effect resistances. Not every status effect has a resistance; for example, Attribute Down effects do not have one. The list of status effects that have corresponding resistances are the ones that can have Immunobreaks applied to them, plus light-aligned sleep (which is a separate resistance from dark-aligned sleep). My guess is that light-aligned sleep would also be eligible for immunobreaks if there was an enfeebling magic version of it, but since there isn't, it's the status with a corresponding status rank that cannot have immunobreaks.
These resistances work much like elemental resistance ranks, so I'm going to call them status ranks. There's 15 levels going from 1-15 (or 150% to 5% if one wanted to match them with resistance ranks), with the higher the rank, the more magic evasion a mob has to that enfeeble. Of particular note, enfeebles that have corresponding status ranks completely ignore their elemental resistances in favor of their status resistances. For example, Ice Threnody helps to land Distract (as it does not have a corresponding status rank), but Earth Threnody does not help to land Slow. For normal monsters, status ranks almost always follow their status effect's element (with the exception of the two sleep resistances, which sometimes mix it up), but NMs often have customized status ranks, such as a much higher silence resistance than wind resistance.
Immunobreaks are possible when a mob has a status rank of 9 or higher, and reduce said rank by 1 stage with each application (2 stages with Chironic Hose). Mobs that have status ranks of 14 or 15 cannot have that enfeeble applied to them until immunobreaks bring them to 13 or lower, no matter how much magic accuracy the caster has. This is why after 1 immunobreak w/Chironic Hose, Elemental Seal guarantees that an enfeeble lands on any enemy that doesn't completely resist that status effect. Landing an enfeeble after immunobreaks resets the mob's status rank back to the original value, and lowers the chances of landing further immunobreaks after it wears.
This system is not used by players at all; players go by their elemental resistances for all enfeebles, which means that adjusting elemental resistance does impact enfeeble land rates. Barstatus spells seem to simply add magic evasion for the corresponding status effects.
Odyssey Misc
Nostos mobs in Sheol B gain +2 resistance ranks from their family's normal ranks, while normal mobs in Sheol C gain +4 resistance ranks. It is unknown if Sheol A mobs get a boost, if this also applies to Mimics/Agons/NMs, or if mobs also get status rank boosts as well.
Each T3 NM gains +4 to each attribute and DT-1% per vengeance level.
Magic Accuracy and Hit Rate on High Level Enemies
The twitter post linked lists a number of required magic accuracy levels to get capped hit rate on various mob levels 124-139, based on their elemental resistance rank (and possible also for landing status effects with their status ranks), where total MACC = Magic skill + magic accuracy from gear and buffs + magic accuracy skill + dStat adjustments + the innate MACC of the spell you're using. Red values are confirmed, black values are estimated.
Some examples of innate MACC are:
TI Nukes, Slow: +0
TII Nukes, Slow II, Bio: +10
TIII/IV Nukes, Addle: +20
TV Nukes: +25
Poison II: +30
Distract/Frazzle: +150
Dispel: +175
Stun: +200