I agree that they are made suboptimal for some reason. Although Astarion (or Laezal) compensate with the intellect band. The optionbto go eldrtch knight or arcane trickster with a dumped down int, is really unbalancing.
On the other hand, classes like cleric or wizard have no magic object that bump a relevant stat. I suppose that neither limit the intellect band or add other ítems. But until then, it seems that Shadowheart would suffer more than Astarion from this.
