What do you do when you see a novax? Rush to preemptively shield every minutely valuable thing in your base. HQs, mexes, power, storage, exp's being built.
What do you do when you see t3 arty up? Shield your power.
Novax is OP by virtue it can instantly change target with 100% accuracy. It will find a gap in your base and exploit it.
Arty while still annoying, takes time to aim at something new, fire, and maybe hit that target. You're not aiming at mexes with T3 arty (unless you basically already won and have nothing else to hit).
You have time an opportunity to shield your base as needed when defending against arty, and generally a lot less of that with a novax.