Zable and Koren both seem to cast Death spell randomly in their routines and are at too high of a level to negate with a level. Get hit with death spell = no bonus. Not to mention the occasional boulder getting off roulette and the like.
I think you're overestimating HP+ gear. In general by the end game I am close to max HP (700-800+). An average enemy hit does around 120-170, 100 hp is less than one hit. The difference between high def and low is like 50-80 hp per hit.
Early - Mid game - Heal Light casting is too slow, the main criteria in a boss fight is making sure your healing stocks last through an entire fight. Most often the difference between medium and light defense by feel is that a light defense hit takes me down to almost nothing, and I need to blow 3+ round drops to take another hit. Medium defense, I might only need 1 round drop before I die in the next hit. The difference in item and time efficiency is the deciding factor, not how many hits I can take in a vacuum.
The problem with resist gear is that it helps you against one element of damage and increases the damage you take from everything else. Wearing Dark resist gear against Gorza for example is counterproductive, you take maybe 25% less damage from dark spells (due to lower base magic resist), but the trade off is that his physical attacks and ghost road now one shot you. Resist gear is only good against bosses (because regular enemies don't use elements too much, and the ones that do use AI to target your weakness specifically) and only against bosses that don't have non elemental attacks. Admittedly a lot of God Beats are kinda one trick wonders, but there's a quite a few physical attackers in the mix. And it seems like you're probably better off using Remove Weakness or More Magic Defense for those God Beasts anyway.
I didn't like undead armor because end game enemies deal way too much damage to fight comfortably. Even with max defense Duran is taking 100+ damage a hit. Evade stacking feels way better than defense stacking and the lack of evade on the chest piece just makes it not worth it. Knowing that max evade is 90 changes things a lot, though i feel like 1% is wrong because I still got hit a lot at 127 in the Dragon Hole (feels closer to 5% to 10 %?) maybe enemies just attack a ton. It seems like in mid game, wearing undead armor meant that I could just about comfortably tank an entire screen of enemies, get really close to death, but get the free heal after. This is extremely relevant in the snow fields where you can't wear pure physical defense because of mages or in the wind area because enemies inflict status effects like crazy. In the end game, I can't rely on using pure defense to last me through a map with undead armor because I will die before I can even take out one enemy in melee, so I prefer having access to regular healing.
For weapons, I think its quite clear because weapons use the same statline for every character despite being better/worse for each character due to their role. Duran's mp drain weapon is pretty bad for example, because he doesn't use spells very much in any of his classes. That one can probably be bumped to medium-heavy for him because of the relative lack of utility. Hawk can't get any innate healing spells, so his strong Heal Light item should probably be buffed to compensate.
The random chance to inflict poison weapon seems really bad. Random amount of hits it takes to inflict status, you just run away? Even though the AI isn't capable of carrying out that strategy? Same with random chance to inflict silence - there are only a few enemies that I want a random silence on (bees because they buff before the FST and eyes because they don't cast often) - everything I else I want to silence asap or I don't care at all. I feel like those could probably be increased to medium attack but with lower crit chance or something.
Armor wise, I think all of the resist status stuff is too weak, feel like it should be buffed to medium to make it competitive with Wendel armor. I already said my piece about resistance armor. Attack up stuff in general don't feel worth it right now. I wish there were a set of armor that negates enemy elemental resists, a weapon that negates evade, and a weapon that can do knock back consistently.