Wearing a resistance item isn't a guaranteed save, or even a 2/3rds save. Especially mental. Dispel, for instance, behaves really differently with saves. Each affect on the affect list is the target of the spell, and each get their own save check. And if I'm not mistaken, the level of the affect is used in the saves check, not the level of the character.
As I said before, not all vulns and resistances are equal. Fire vuln, ice vuln, water vuln coverage... You can expect those items to likely be disallowed to the races that have them by now.
The back brace is also disallowed to classes that are specifically designed to have trouble with lightning spells, like crusader and druid.
I personally don't see us doing the same for mental resistance items, though. There's not much in the game in terms of mental damage, leaving mostly mental spells, the most prominent of which behaves very differently, and thus is fairly irreverent of mental resistance. Mental resistance is honestly much less powerful than fire, ice, or even lightning resistance.
Poison resistance is also a weird one. It's a double edged sword. And how it works was changed after some testing proved it's interaction was weird. It used to not only make you harder to poison, but also prevented the poison damage (barring high level poisons like lotus scourge and shaman poison) because of MUD math. Basically, everything is rounded down, there's no half integers. So the spam of 1 damage poison drains was getting reduced to .5, which the mud interprets as 0. This interaction was changed, though. It no longer has this upside. Which puts it right back to a mediocre-at-best resistance.