From memory ships can use the Bussard collectors to collect hydrogen
Which, funnily enough, is just backstage doubletalk and not really part of the onscreen reality of Star Trek. There are two things these things are known to be capable of doing: venting out
hydrogen, and scooping up random dirt from a dense nebula. For all we know, they can only work efficiently in dense nebulae, or perhaps underwater, and will achieve nothing in the thin mists of interstellar space. Or perhaps they are only intended to remove gaseous obstacles from the path of the ship, and can do nothing with the collected gas except vent it out again?
even something as complicated as a pocket watch might actually require some assembly of replicated PARTS rather than pulling a finished product out of the slot
Possibly so. But our heroes and villains seem to get fully functional weapons out of replicators easily enough. Perhaps the projectile rifle from "Field of Fire" did require some assembly we didn't witness - but the multi-phaser ordered up by Gul Dukat's defense program in "Civil Defense" emerged quite ready to fire...