While we're here, Battlestations and Citadels that have Engineers on their build roster never seem to construct them. Support Factories do. (Support Factories are amazingly good with their fleets of Combat Engineers. I find these to be of much greater value than Battlestations and Citadels.)
The unique selling point, as far as I understand it, is that Battlestations and Citadels can build turrets on worlds not owned. Aside from BSs and Cs never building their Engineers to assist in such construction, I've never really found that ability to be all that useful. It's not a terrible ability, and I'd much rather they retain it than lose it, but it does feel like BSs and Cs aren't especially valuable and I'd never go for one of those if getting a new fleet is possible (or a Support Factory).