[Roanon]

PvK said:
Short- to mid-term, rather than very short term. The reason is that more of your shipyard time and resources are going into working on ships which aren't ready to fight

Better than shipyard time going into ships not ready at all.
I usually do a 2-step retrofit, starting with a roughly 50% cost hull needing 2 turns to build, and going up to 70% then 100% in 2 steps. This produces a ship that would need 5 turns to be built completely otherwise.
If I need 2 turns to build a hull plus 2 turns to retrofit and repair, rather than 5 turns to build, I have MORE ships - short, mid, and long term. The real waste of shipyard time is continuing to build a complete ship turn 3, 4 and 5 instead of already building a second shell, and not using retrofitting which is unlimited and in addition to any shipyard capacity.

1) Retrofitting costs a significant amount more than building components on a ship in the first place, and they start out damaged, meaning even some of the upgraded ships have unworking equipment and/or extra repair ships and bases need to be built and maintained and/or ships are at planets instead of on the front lines.

10% extra cost for retrofitting. If you bild a ship with 50% of the total costs and retrofit it, this is a total of 5% extra on top of the total costs. Yawn.
Ok, you need some starbases with repair bases. Well some extra costs for that. If you are THAT short on resources, ok, do something else. Like looking for a way to earn more resources .
And for the extra time to the front: the opposite is true as you cannot build every ship near the front (usually, the front planets have other things to do than building ships, like resource mines for example...). With the retro-technique, you build far from the front, and already send them towards the repair/train base (which should have a convenient location) during the 2 turns while they are being retrofitted. So after 4 turns you have a completely trained ship near the front rather than an untrained ship 1 turn later and far away from the front. Which costs a lot of maintenance until ready to fight.

2) A large part of a ship's cost is in its engines and control components (one of the reasons why big ships are dominant in the unmodded game), and these need to be built on the shells. So there is a lot of cost and maintenance going into ships which have little or no combat strength. Ships that are being fully built don't cost any maintenance until they are complete.

The cost to build the engine and control components on a ship shell is not different from the cost to build the same parts on the complete ship which is ready much later. Absolutely no extra cost here.
And it is in fact CHEAPER to train an half-finished ship for half maintenance rather than a fully built ship for full maintenance.

All in all, it never depends on the situation, this strategy is always good as soon as you use battleships or better. The resource cost is actually lower, not higher, as you do not have to waste much maintenance costs for training a fully equipped ship. The overall fleet strength is actually higher, not weaker, as you have fully trained and equipped ships much earlier then when building complete ships.
Ok, you have a weak fleet the first turns - but with the standard build schedule, you have no fleet at all these turns.

You must have A LOT more resources and shipyard capacity to beat this retrofit strategy with standard complete-hull building.