2 reasons.
Number 1 you can farm all the resources needed to build the drifter weapons in duviri. You don't need to unlock Jupiter for a neural sensor or Saturn for good amounts of plastids...
Number 2, the mastery rank is 0.
I guess 2 drawbacks would be having to fight the orowyrm so many times to get his unique resource drop. 5 times to get the 50 required for a drifter melee weapon. Also, the stances. The only stance i can see dropping early game would be the hammer stance and you'd have to farm Sargas Ruk to get it.
Oh yeah. As far as the Sun & Moon. After the beginning duviri quest, if you go back to duviri, you should be able to get enough resources to build Sun & Moon in just one more run. You get the stance for free so that i think is definitely the new best choice for a starter weapon.