I think these are fairly neat ideas, and I can actually see circumstances in which they might be worth the inventory slot! If I may, repurposing these items as weapons might be worth considering, as that would make it easier to equip/unequip them on the fly to apply their stat boosts and penalties as necessary.
These sorts of “bonus with an equal penalty” trade-off skills are primarily useful for units who really want to laser-focus on filling a certain role, which is also important to bear in mind. I don’t think it being a “net gain of +0 stats” is necessarily the issue, because, say, pretty much any physical unit who’s going to be tanking other physical units is going to greatly prefer 5 more points of Defense over 5 points of Magic. A magical attacker who’s not going to be doing a whole lot of defending is going to like 5 points of Magic more than 5 points of Luck.
Just some food for thought regarding this! As with anything, how well it will work depends as much on how the game is balanced around it as on what “it” itself is!