The Aeon Sacrifice ability is criminally underused, and I think that's a shame since it really adds to faction diversity in an area where there otherwise is none: engineers. While I've seen that SCU sacrifice is being integrated with the Rapid Fabricator, I really don't think this will increase the use rate much.
Here is my proposed rework:
Instead of directly funneling the mass/energy cost of the unit into the structure, the unit will begin to build at 3x its usual build speed. The engineer will shed health over the course of the duration, and it can only be cancelled by the premature death of the engineer. If the unit is not killed, it dies and refunds 40% of its cost directly into storage.
For T1-T3 engineers this would last 10 seconds, allowing them to do 30 seconds worth of building before death.
For SCUs, it would be 1 second per 150 mass cost of the SCU. Which would be 19 seconds for just Rapid Fabricator and 43 seconds for RAS. This is about 33% less efficient than the Engineer numbers.
Let me know what you think! I'm especially unsure about the resources refund, but I thought getting 40% of a RAS com back after a 43 second long channeled ability shouldn't be too overpowered.