I want to address this with some maths.
We will be making the following assumptions:
A DD does not put points into CHA while leveling. Generally speaking, a DD character will put points into STR as they are a melee martial character who wanted that dragon spice in their life. This means that the worst case is that a 10th DD will have 8 CHA score / -1 CHA mod and be able to get that up to +5. At best, a 10th DD will probably have about 18 CHA score / +4 CHA mod and be able to get it up to a +10 modifier. This means that the DCs can range anywhere from 25 to 30.
A full caster will have a score of 22 in their governing attribute (Charisma, Wisdom, or Intelligence) by 16th (Assuming they only have one caster attribute to worry about). This comes out to a +6 modifier. With their whole magical bonus, this puts them to a score of 34, or a +12 modifier. The DCs for their spells are calculated with a base 10 + spell innate level + Caster Attribute Modifier.
With this, all spell DCs at 16th caster level are as follows (From Innate level 0 to 9th; ignoring feats and other special bonuses): DC 22 at 0 (Cantrips), DC 23 at 1st, DC 24 at 2nd, DC 25 at 3rd, DC 26 at 4th, DC 27 at 5th, DC 28 at 6th, DC 29 at 7th, DC 30 at 8th, and DC 31 at 9th.
By comparison, the breath ranges anywhere from a 3rd level DC spell to an 8th level DC spell on a full caster with none of the advantages. The breath has a terrible save type (Reflex) which means it can be further negated by evasion. Most of the damage types that are available are pretty well resisted generally speaking (Cold, Fire, Electricity). The average damage (8d10) of 44 means that energy buffer will more often than not just negate most of the damage anyways disregarding evasion. Also this feat doesn't discern friend from foe making it difficult to actually use in melee with your other melee friends there.
Personally, my quick and dirty fixes would be allow a choice of scaling type at breath weapon unlock; give both a breath weapon that scales with CHA and a breath weapon that scales with CON. Make it so at max level DD, the breath no longer hits friendlies, or even just make that an optional bonus epic feat for DD.
My lengthy, probably balance-breaking idea is to buff the breath weapon then give it limited charges that don't come back until after a rest. The DC would be the same calculation and have an option to choose between CON or CHA scaling upon acquiring the breath weapon in DD, but now the damage would scale similarly to how spells scale on casters up to some maximum bonus damage at 10th DD where it also no longer harms allies. This would make the breath a menacing threat but also a limited resource the player has to manage.
We will be making the following assumptions:
- The characters are level 16.
- The characters have maxed out their magical bonus to any attribute that governs their DCs.
- The DD character is 10th level DD.
- The caster character is a level 16 full caster with no PRC.
- The characters have no racial bonuses to any of their attributes.
- This is pre-epic levels (Levels 1 to 20)
A DD does not put points into CHA while leveling. Generally speaking, a DD character will put points into STR as they are a melee martial character who wanted that dragon spice in their life. This means that the worst case is that a 10th DD will have 8 CHA score / -1 CHA mod and be able to get that up to +5. At best, a 10th DD will probably have about 18 CHA score / +4 CHA mod and be able to get it up to a +10 modifier. This means that the DCs can range anywhere from 25 to 30.
A full caster will have a score of 22 in their governing attribute (Charisma, Wisdom, or Intelligence) by 16th (Assuming they only have one caster attribute to worry about). This comes out to a +6 modifier. With their whole magical bonus, this puts them to a score of 34, or a +12 modifier. The DCs for their spells are calculated with a base 10 + spell innate level + Caster Attribute Modifier.
With this, all spell DCs at 16th caster level are as follows (From Innate level 0 to 9th; ignoring feats and other special bonuses): DC 22 at 0 (Cantrips), DC 23 at 1st, DC 24 at 2nd, DC 25 at 3rd, DC 26 at 4th, DC 27 at 5th, DC 28 at 6th, DC 29 at 7th, DC 30 at 8th, and DC 31 at 9th.
By comparison, the breath ranges anywhere from a 3rd level DC spell to an 8th level DC spell on a full caster with none of the advantages. The breath has a terrible save type (Reflex) which means it can be further negated by evasion. Most of the damage types that are available are pretty well resisted generally speaking (Cold, Fire, Electricity). The average damage (8d10) of 44 means that energy buffer will more often than not just negate most of the damage anyways disregarding evasion. Also this feat doesn't discern friend from foe making it difficult to actually use in melee with your other melee friends there.
Personally, my quick and dirty fixes would be allow a choice of scaling type at breath weapon unlock; give both a breath weapon that scales with CHA and a breath weapon that scales with CON. Make it so at max level DD, the breath no longer hits friendlies, or even just make that an optional bonus epic feat for DD.
My lengthy, probably balance-breaking idea is to buff the breath weapon then give it limited charges that don't come back until after a rest. The DC would be the same calculation and have an option to choose between CON or CHA scaling upon acquiring the breath weapon in DD, but now the damage would scale similarly to how spells scale on casters up to some maximum bonus damage at 10th DD where it also no longer harms allies. This would make the breath a menacing threat but also a limited resource the player has to manage.