People sometimes forget to consider in their paper napkin calculations for mass cost/mass generated benefit, that this is a unit that can also build shit, and in a pinch, fight. IMO that's part of the reason it works AND its difficult to punish if implemented well. You are scaling eco and buildpower at the same time, and have a unit that can be upgraded in a pinch to serve as a fairly mass efficient combat unit. So in reality, it pays for itself WAY sooner then you think it does, if you also factor in the ~2k mass of combat unit, 800 mass of build power, just out of the gate.
That and its stupid convenient to spam on repeat.
I think I remember a few years back Icy made a mass fab/gateway template and the mass cost for a RAS SCU can be stupid low if you pack enough mass fab/mex adjacency on it. Problem is spamming multiple gateways and producing unassisted. Takes too long to pay for itself, but was cool in concept.