MeleeATK/MeleeDEF effects both bypass a target's Magic Immunity (ImmuneToMagic) even if the effects themselves are Spells and, well, Magic.
This means, for example, that you may be fighting an Obsidian Golem w/ around 40 DEF as well as Magic Immunity and yet, it will have 0 DEF after hitting someone with an Ereog's Token, despite Curse being a Spell to which it should be immune. Basically, you just went around the Magic Immunity and removed all its Defense, by virtue of having a MeleeDEF Item. That's obviously only one example to illustrate the issue.
A Graveseal MeleeATK/MeleeDEF Item will also bypass Criticals Immunity (ImmuneToCriticals) and I suppose this may extend to other immunities as well.
LH Mods by Primal