How do you deal with badly made STL files?

Some STL files print perfectly right out of the gate, and others are just a complete mess. Thin walls that won’t slice properly, non-manifold geometry, random holes, weird artifacts that only show up halfway through the print.
Sometimes auto-repair works, sometimes it makes things worse. I’ve tried fixing stuff in the slicer, running it through Meshmixer, even opening it in CAD and patching it by hand, but that takes time and doesn’t always feel worth it.
At what point do you decide to fix the file versus redesigning it from scratch or just moving on and finding a better model?