You only need 1k of mass to get enough e storage to overcharge a harbinger in one shot. That is undeniable. How much mass does it cost to get the power that you need to fill up your power bar? It depends.
It depends on whether you get overflow from teammates. It depends on whether you already built power in order to achieve something else. Maybe you built an extra t2 pgen so you would have enough power to get Seraphim's first nano regen upgrade. If you already overbuilt power, you don't need to spend any extra mass on it.
You can get the power by harvesting it from trees.
If you want to build pgens to get enough power to overcharge harbingers, how many pgens do you need? Well, if you fire an overcharge that costs 18400 power, and you want to replenish that for a second overcharge, how many pgens do you need? That depends on how quickly you need it to regenerate.
If you want to spam manual overcharges, you have 4 seconds to come up with 18.4k power, that's less than 5k power/second, which means 2 T3 pgens would be sufficient. For 7480 mass (the cost of 2 T3 pgens + 4 storages), your ACU can become a death god that kills 210 mass/second of harbingers (and that's assuming you only kill 1 at a time, if two or three are clumped up, the rate doubles/triples). 210 mass/second is like having 8 storaged T3 mexes. If you want to be able to deal damage destroying 210 mass/second of your opponent's units, it's total reasonable that you would have to invest 7480k mass up front for that. If you are blapping harbs non-stop, the investment would pay for itself (meaning you break the same amount of your opponent's mass as you spent on mass to get it) in less than 36 seconds, and that's not even counting the reclaim you would get if you're able to stand your ground and reclaim the battle site. It doesn't count whether your other units are able to get favorable trades because they suddenly outnumber the enemy harbingers.
If you invest significantly less, let's say 8 storages (2k mass) + 2 T2 pgens (2.4k mass), that's a total cost of 4.4k mass. You would be able to overcharge harbs twice without refilling your storage, and it would take 18 seconds to refill the storage enough to do it again or 36 seconds to fully recharge storage. So in the span of 40 seconds, your ACU could one-shot harbs 4 times. That's 3280 mass worth of enemy units killed (more if you can OC multiple harbs at once, which is not unreasonable, if the enemy sends harbs at you in small numbers that would be suicide). For an investment of just 4.4k mass, in 40 seconds you can kill 3280 mass of enemy units and maybe scoop up 2400 mass of reclaim from those kills. The idea that overcharge is nerfed to the point of being uneconomical is just not true.
A harbinger itself costs 9.6k power just to make and only 18.4k power to one-shot. If you overcharge 2 harbingers at once, you literally spent less energy on that OC than your opponent spent to build them in the first place, and you spent zero mass. What about the fact that your opponent needs to build T2 pgens just to make harbingers in the first place?
And none of this math takes into account the value of having such an ability on your ACU: it helps you to survive land-based attacks/snipe attempts. And the fact that you can position your ACU where you want it, to do the most damage. Your ACU might have massive shielding, big regeneration, or stealth. You can have mobile shields covering your ACU. You can put all of this firepower anywhere you want on the battlefield, you can even drop it from a T2 transport. It's not like investing in point defense that will only protect 1 area or artillery with limited range. Your ACU might have T2/T3 upgrade so it can lock down reclaim fields and quickly scoop mass.
Compared to your other options in this game, overcharges are still massively overpowered in terms of the damage you can do, for the amount of mass/energy you need to invest. That doesn't mean they need to be nerfed further. You have to risk your ACU to use them, and they serve a valuable purpose of making it hard to kill the ACU. They play an appropriate role to improve gameplay/game balance.