The Cursed Dragons were a species of magical beasts that Heaven and Earth had punished due to the limitless potential of their flames. Noah had inherited that ability when he fused with one of their specimens, but he didn't rely too much on his fire since he often had stronger attacks available.
It merely was a matter of energies. The primary energy contained inside Noah's lungs fueled the flames and made them a powerful tool that he could use repeatedly. Still, they didn't match the might of his martial arts.
Noah would use his "Breath", his inscribed weapons, and his physical strength when executing his forms. The sheer power of his lungs couldn't match the energy released when he made use of all the tools of a cultivator.
His fire wasn't weak. It merely appeared limited when compared to all the abilities that a hybrid could wield.