Increasing a character's stats past the baseline value of 10 should have only positive effects on gameplay and combat. Following this is good game design and makes sense. In PoE, there are many abilities that have drawbacks such as afflicting the character for a duration or forcing them to take damage. In all of those scenarios I've found, the following things happen with regard to self-harming effects:
Duration increases with the player's INT, beneficial effect increase (like from Strand of Favor, which contradicts its description), and Power Level (if class ability).
Damage the player takes