Moonblades aren't artifacts, they're listed under "Sentient Weapons". And even after looking it over, a +3 weapon that deals an extra 1d6 of fire damage to one type of enemy is little better than a standard Dragon Slayer sword. Worse, actually. Yes, it has a +3, so he likely will not be missing often, but the dragon slayer does 3d6 extra damage to the dragon. In fact, the Holy Avenger itself is a +3 weapon with an extra 2d10 Radiant Damage on all Undead and Fiends. It also glows, and gives advantage on saving throws, so technically, it's better, except it's not sentient.
And you could always sheath the weapon when it starts glowing. That would stop the glow. If it's a blade, that is.
Oh, and yes, you're going to be getting artifacts. This is my "EVERYTHING IS HERE" game. They're all in different places, but completing missions will be a very, very good thing for you.
Just don't complain when I sic a Dracolich on you.