I'm not fond of how they do nothing for someone already high in that stat. Could always make them a sliding scale to eliminate that aspect, and reduce or prevent min-maxing:
If you have a 10 or less, item gives you a 15.
If you have 11-14, item gives you a 17.
If you have 15+, item adds +2 (max 22).
This makes them much more valuable, but taking up an attunement slot is a huge balancing factor.
Since that makes them always useful, you'd be less likely to see a a character using an off-stat item because the other party members it's relevant for have a 19 or 20 already, but I don't imagine that was very common anyway unless the character's not got any other attunement items.
If you deliberately built your warlock with an 8 CHA because you knew you'd get a 19 CHA item, well, that's powergamery.
I agree in general, but I have been in several games where the starting conditions were along the lines of "We're starting at level 5, you have 1000 gp, any mundane items you need, and one uncommon magic item of your choice." In that environment, picking up something like Gauntlets of Ogre Strength is a sneaky good choice for classes with multiple ability score needs like a Paladin, or adding on a nice melee attack to a Cleric or Bladelock.Sure. But that's not how it's supposed to work - no magic item shops in 5e. So they don't use "I can pick the perfect item for my character" as a factor in determining rarity.