Novax isn't too powerful on most maps, but it is actually very, very good on setons, and probably similar maps with spread out bases and lots of mexes. Just do the math:
Let's say you are the air player and have 13 mexes to defend. Only the core 4 mexes can possibly be defended with less than one shield per mex, even assuming 1 is sufficient for those 4 mexes giving a total of 10 so far, you'll still need a bunch of extra shields if you don't want your air grid to explode, plus your smd in the middle. So you'll still need probably at least 13 t2 shields (with maybe a couple t3 shields on the air grid because you need their longer range). If they are sera shields, that's 13x700 mass, equaling 9100 mass, PLUS you will need an extra t3 pgen for power to run those additional shields which drain 13x250=3k power. One t3 pgen, which won't even totally cover that power drain, is 3240 mass. That puts us at basically 12.5k mass spent (rounding up since we still haven't covered the full power drain) on countering the novax. BUT THIS IS JUST ONE PLAYER! There are 4 players that all have to do the same thing or the novax will just target them instead. Are you able to multiply by 4? Now the enemy team has had to spend about 50k mass (12.5k times 4...) because all the other slots have to build very similar numbers of shields, PLUS a bunch of apm queing up engies to build shields on every mex plus every important structure not otherwise covered by a shield (usually you don't have every pgen or hq right beside your mexes) to avoid significant damage from the novax. Some spots like the corner trio of mexes for beach you might get away with fewer shields, but you'll probably need a couple extra shields somewhere else that aren't on mexes so I think it basically evens out and this is a pretty fair estimate. Even if the other slots can get away with just 10 t2 shields (so we save 9x700=6300 mass) it's still going to be well over 40k total mass spent. But the novax STILL gives very nice scouting, PLUS it can STILL be used on any units not shielded, such as navy or t4s, or landed asfs, sams, etc. And it can help to soften up shields for a strat snipe of an smd too. And AFTER the enemy builds all those shields, you could still simply reclaim your novax, which takes a whole lot less apm for you to recover the mass than for the enemy to reclaim all their shields. Even further, you can just start to build the novax and not finish it, and possibly force out a whole bunch of shields in anticipation of it and not even invest much at all yourself.
UEF and Aeon shields are a bit cheaper for both mass and power, but give less range and don't affect the calculations too much. Generally, due to the small range you will need a couple extra shields to protect your buildings if you are aeon so you don't end up saving too much anyway. iirc the lowest tier cybran shield isn't strong enough so you need at least ed2 which makes the cost also similar. If you say about 600 mass times 10 shields for each of the other 3 players, that's 18k mass, plus 9k mass for the pgens to run the shields, so 27k, plus about 11k for the air player's t2 shields. So 38k total mass for a pretty conservative estimate.
Still, I haven't added in any cost of having to build t3 shields to protect the air grid. T3 shields cost a lot of mass. Maybe just 2-3 is sufficient depending on your air grid size, but that's another 5k or more mass you have to spend. So about 55k total mass will probably be spent by the team, and at least 40k. So, even if you use far more optimistic numbers by using other faction shields that are cheaper, it's still definitely going to be cost effective to build at least one novax. Building a second probably might not be worth it, unless you can surprise them and snipe an smd or something before they realize you made multiple and build additional shields. That shouldn't happen, but if there is one player managing multiple bases in the late game it would not be impossible.
edit: so does it need to be nerfed? Well I think it's quite weak on maps where you need far fewer shields, like canis, so it's hard to say overall.