This is one of those situations where Table Top doesn't translate perfectly into a Videogame.
"Set ability to X" items are more powerful/build-defining in Single Player Games with no loot randomization because you can meta-game it much more easily. The other issue is due to a design choice by Larian. With no item attunement system in place, the game will revolve more around magic items because they can contribute so much more to a character's power.
It probably won't break the game for most normal players, but we'll probably see a lot of min-maxers or speedrunners dump certain stats and rely on these items instead. This existed in BG2 also (ring of human influence, STR belts), and I don't think it was too disruptive, but it does force optimization towards certain metas.
Yep. Since Larian are so much about "what works in a videogame" they should identify these kinds of items as something that don't.
Unlike tabletop, the headband is always available and it's going to be part of builds. And the stupid kind of builds that no sane person would ever make in tabletop like the 8 Int Wizard. And then a stupid build becomes OP, and it's just... stupid.
I'd much rather play a High Elf EK who is naturally intelligent, and could use such an item to enhance their ability, not replace it or to become on par with another EK with 8 Intelligence (who of course would also have higher stats elsewhere).