First not an issue in the games I run, I will just never hand them out. Great thing about magic items that you don't like as a DM just don't have them in your campaign world.
In the games I play I find them horrible.
Strength based character, spends ability increases to boost his strength to a 20. Say he starts with a 16, that takes him two of his increases, that he could have spent on another ability like Con or gotten two nice feats that would give him more options. The party acquires a belt of hill giant strength, it goes to the cleric or other melee type and the fighter feels jipped he didn't take those feats or Con points because if would have taken them he would be best served by the belt.
Now the same party instead finds a belt of frost giant strength, it goes to the fighter and sure he is happy for the extra +1 to hit and damage, until he realizes he still wasted those ability score increases, if he had chosen feats and his strength was still a 16 the belt would still give him the same bonus.
Finding a magic item should never make you regret your previous choices, especially if those choices can not be undone.
But in the end, I find strength build characters boring and mostly play dex focused or spellcasters so I doubt I will ever feel like I wasted my ability score increases, just feel bad for those who do.