I wanted to build one but settled for a 500m variant. Its hard to make the outter aesthetics match that of a borg cube and if you were to fill its subsystems, thats around 125 million blocks for just the 500m version. A 3km version is 2.7x10^10 blocks, and it might not even update correctly when using adv buildmode due to the sectors. even if you place everything using 10x10x10 cubes thats still 27000000
another issue is that theyre undockable because the docking beam cant extend that far out. so if you build it, you need some sort of different protection system or build it hidden. The Huge mass of the cubes makes them lag fairly badly. I just laid the base hull and when i entered my smallship to fend off a pirate it was very laggy.