Ah things I would like to point out. Noble phantasms do consume mana. Nidhogg using one of such power and scale WILL drain his reserves. If he was a phantasmal species he would have EX ranked mana to balance this out, but I'm not sure we can classify Nidhogg as one. Medusa is classified as a demon, and so would Nidhogg considering how similar they are. The 666 Beast is also classified as a demon. Unfortunately, demons outside of True Demons lack the same stat capacity as phantasmal species do. However, Raijin has given Nidhogg prana burst(?) which is unique to phantasmal beasts, so I am thinking he has Nidhogg pinned as a phantasmal beast(???).
Also, being summoned in the Holy Grail War does limit heroes in stats. There is a limit to what the grail can summon, hence why Heracles doesnt move mountains and gouge out chasms with each and every attack. Hence why Karna doesn't punch people with enough force to "shake the entire universe"(Yes, this is what happened in the Mahabarata).