Most free STLs fail because the walls are too thin
I keep seeing the same print on hobby forums: someone downloads a "lighthouse" or a "boat," hits print, and gets spaghetti at 12 mm.
It's almost never the printer.
A lot of marketplace files are modeled for resin or for a 0.2 mm nozzle. On a $200 FDM machine with a 0.4 mm nozzle, any wall under 0.8 mm is a coin flip. Overhangs over 50° without supports are a coin flip. Tiny masts and railings are decoration in the screenshot and trash in the bin.
What I do before I sell a file:
Every wall is at least two line widths.
I print it on an Ender-class bed, not just a CoreXY.
If a piece needs a brim, the file notes say so. I don't hide that in a 14-page PDF.
If you're printing coastal pieces — buoys, dock hardware, lighthouses — start with one calibration model, not the whole scene. Get first-layer adhesion right. Then scale.
That's the whole method. Fancy slicer profiles don't save a file that was never meant to print.