Installations are not practical for any body where the colonization cost is big and aren't meant to be. Perhaps the values could be juggled some, I don't know. But they are useful in early games for low colony cost bodies like Luna and Mars, because civilian shipping lines will take care of transporting enough colonists and infrastructure to keep them going. Especially if you don't need colonists to work mines on those two. So you don't have to waste resources/time on building OHs or expanding your shipyards. Usually by the time I have the spare industry and SYs to start constructing orbital terraforming bases, Mars and Luna will both have been terraformed already, at which point the installations can be moved to outer system moons or off Sol with shipping lines again. This way I don't have to worry about keeping a fleet of tugs in service and there is an alternative terraforming method always available as necessary.

Maybe it's different for TN starts but I've found this a very workable strategy in conventional starts.

Or even without habitats... one arge civilian yard isn't that expensive. Prefabricate the modules with constuction factories, and the terraformer builds in no time at all - even if you add a few also-prefabricated engines.

I never build the installations. Even if they were significantly cheaper, the manpower requirements are the dealbreaker.

For one, because servicing such large ships requires a different kind of economy. You need a shipyard for tugs, then there is the fact that constructing these ships is going to take a very long time, because your factories will have 12k or something like that, production, while your shipyard will have 1k.

It is an economic and industrial question, rather than efficiency. Do you engage your military hard and limited assets, or your economy? It's that kind of thing.