I think they nerfed breaths against NMs because they ignore usual defenses. High DEF and VIT give NMs great physical defense and MDB/INT/MND and MDT give NMs great magic defense, but until TOAU, most NMs had 0 defense against breath damage. So in order to keep NM difficulty relatively high, they probably added a breath damage taken-% component to new NMs since no other stat would reduce breath damage(it ignores all defenses). Breath Damage is more effective than magic or physical because 1) breaths don't miss 2) they resist less(i've heard they have fewer resist states than magic, they don't have 1/8 resists only 1/2 and 1/4)). Since breaths do moderate damage but are very accurate and consistent, it would be an easy way to wittle NMs down. This is probably why they nerfed it.

Souleater damage might also be breath damage, I've never tested it but it would be interesting and difficult. Does Souleater add damage even when a mob's Invincible or if you hit for 0?