The Delta Flyer dropping out of warp; note the nacelles retracting.
Ships with movable nacelles (Voyager, Delta Flyer, Enterprise E captain’s yacht) look cool, and it can be a good way to achieve a compact design but still let the nacelles move out for proper warp geometry. But I’ve always worried that too many moving parts could gum up the works. A solid nacelle pylon just seems more sturdy in battle than a nacelle that has to have gears or whatever moving parts would be in there.
So I’ve never used retractable nacelles in my designs. What do you think?