Naaaaaaah, it's way bigger than that.
L2J xml sucks by design (e.g: skill start at level 6, you have to make it start at 1 for useless shit)
Current aCis xml is based on really L2J one which sucks even more.
Beside that :
L2J targeting is fucked up, not even close to retail and you'll have tons of issues with it if you keep it that way.
L2J stats system is fucked up as well, just check critical calculation , or your given exemple on resist and it's mainly stats calculation, not really formula
The fact is not effects based, most of them sharing the same code base, with shitty add/mul/blabla which makes it easy to makes typo
L2J made a lot of retail conditions simplification, there is even missing one, and doesn't handle all type of it (target, passive, regular)
and that's just a really quick sum up. You have no idea how many dirts we found when we started this quests with L2J data.