Are mining ships and fuel harvesters useful in any situation? I find its much more efficient to create mining/harvesting stations so you can build them using construction factories and then just use freighters/tankers to ferry the goods from those. IMO the mining modules are just too big to ever be efficient on a ship and having gas stations scattered around is pretty useful.
The later parts have been answered but no one has answered this part.
The main reason to use mining/harvesting ships instead of platforms is logistics: a ship even with very minimal engines can relocate itself, slowly but surely, without the assistance of a tug. The flip side is that since those kinds of ships move so rarely, the engines are largely wasted tonnage the other 99% of the time, and a ship with one or few engines will move so slowly from one mining site to the next that using a tug is almost certain to be more time-efficient (but not fuel-efficient, if that's a concern).
Logistically, especially for large empires with many dispersed mining nodes, tugs do represent certain costs, both in resources (fuel but more so gallicite for what is basically a mass of engines with a towing winch on the back end - commercial engines are cheap, but building three dozen of them can stack up costs) and complexity as a large empire has to keep its tugs in the right place to minimize downtime.
Finally, being built from a shipyard is not necessarily a downside for a mining ship. Building mining platforms with industry means you are not using your industry for other things, while a shipyard can only be used to build ships and the only demand on planetary industry is the cost of that yard (usually this is less than even a single mining platform in net BP). I think most players with large empires and a lot of shipyards will usually maintain at least a couple of massive yards that can slowly put out a stream of 250,000-ton mining platforms regardless of what the planetary factories are needed for at any time.
As an aside, NPRs as far as I know only build mining ships, never stations, but I think this is mostly because they have not been taught how to use tractor beams yet.