Are you trolling, or just incredibly stupid? Ras sacus are actually basically just as efficient as fabs when you factor in their bp and dps; everyone knows this. Both can pay for themselves in just over six minutes. From many months ago on the old forum:
As some have already done, looking only at direct mass costs, an SCU costs 6500 mass to produce 11 mass/second, which gives a payback time of 590sec (9min51s).
A MassFab costs 4000 mass to produce 16 mass/second, which gives a payback time of 250sec (4min10s).
Except that MassFabs also need a constant supply of power -1500 power/second, which is provided by power generators that also have a mass cost. A T3 power generator produces 2500 energy/second for a mass of 3240, the energy cost is about 1.3 mass/(energy/s). A MassFab therefore has an additional cost of 1.31500 = 1950 mass from generators that are exclusively dedicated to supplying its energy. This brings the payback time to 372 sec (6min12s). On the other hand, SCU produce 1020 energy/sec, which reduces the number of power generators to be built, this can be considered as a cost reduction of 1.31020 = 1326 mass. This reduces the payback time to 470sec (7min50sec).
Taking the energy into account, a SACU needs only 26% more time than a T3 MassFab to be profitable. If we add the mobility, the tankiness, the builtin turrent, the reduced space consumption and the engineering suite, you explain why it is better to spam SACU than MassFab.
As Strogo said, these calculations somewhat understate how useful fabs are because they do not factor in any adjacency bonuses.
But the buildpower of ras coms is certainly quite important as well, so it's important to quantify that. They provide 56 bp, which is equal to almost 2 t3 engineers, or slightly more than a hive upgraded to the second tier. So that is worth about 600 mass alone, meaning they will pay for themselves almost a minute faster, if you value the bp. Even if you are not using the bp constantly, it is still worth a significant fraction of that number.
Their gun does 300 dps, which is almost as much as two t1 pd. Especially given that sacus are mobile making that dps more useful than pd, we could value that at about 500 mass (the cost of two t1 pd), meaning they pay off another 45 seconds faster (if you value that capability). Obviously ras coms are very rarely used as combat units, but IF they save you from building extra tele def, then you would need to incorporate that value, even if you don't think it's worth quite that much.
Having at least a few sacus just to drop on big reclaim piles is also pretty useful because of their hp.
So if you factor those things in, they are actually pretty close to as cost effective as t3 mass fabs, less any adjacency bonuses, which can be pretty significant. But, I would say the adjacency bonuses are somewhat offset by the sacu mobility, hp, and compactness, so you can protect them all under one assisted shield (which, conveniently, the sacus can assist themselves).
Overall, I think ras coms are superior to fabs, but fabs are a little better if you don't have much use for sacu bp, and aren't too worried about them dying and having to invest a lot to protect them. And still, it depends on the game situation, so it's hard to say one is just better than the other. It can be a good idea to build some fabs for more efficient eco first, then transition to ras coms when their other characteristics become more valuable. Maybe you have plenty of bp and need more eco before you can even take advantage of the sacu bp.
Edit: the combat ability of sacus is very situation dependent, even for the tele def example I gave. If you have 30 stacked in one spot of your base (dealing 9k dps), perhaps the marginal (additional) benefit of more tele def is basically zero, because you kill an acu teleporting fast enough anyway. But if they are teleporting shielded sera sacus to you, and hitting your shields with arty at the same time, maybe the extra dps becomes important to keep your shields from taking too much damage and collapsing.