In the current game balance, using SACUs for their build power is something almost no one does, except for when a player just happened to have a bunch laying around after spamming RAS or a cybran building HARMs. These changes are designed to improve the engineering capacity of SACUs so they become competitive to normal engineers and engie stations, but not to surpass them.
Buff Rapid fabricator BP 42 (98 total) > 94 (150 total). Or in other words, boost mass per BP from 3x times as expensive as normal engies, to 2x times as expensive putting them roughly in line with engineering stations.
Buff Rapid fabricator with + 5 build range to reduce micro and pathfinding problems.
Rapid fabricator is simply in inferior upgrade in every meaningful way. Its less efficient and slower than building engies or engie stations, while also taking up an upgrade slot that could be used for other, often better, upgrades.
Aeon:
Remove Sacrificial systems and add its effect to Rapid fabricator. No change in cost to Rapid fabricator as the aeon player is still technically paying more then they did previously to use the sacrifice feature (800 mass upgrade against a 150 mass upgrade).
(If possible) change sacrifice so that it uses the actual mass value of the SACU (base cost + current upgrades), hardcapped at a maximum of 7000 mass donation. The cap is intended to prevent Aeon Telebuilding from getting buffed too significantly by making the amount of teleport-capable SACUs required to insta build units approximately the same, while still allowing sacrifice to be better in every other situation.
UEF:
Buff C-D2 Engineering Drone. SACU BP 56 > 80. Engineering drones 1 > 2. Engineering drone BP 35 > 30. Buffs total BP to 140, keeping it inline with its current balance of being weaker in total BP but more versatile.
Nerf cost and BT to be inline with other factions engineering upgrades.
Obviously the numbers are not set in stone and as such feel free to discuss them, as well as other changes or comments you wish to add. Thanks for reading.