I frequently use Melodyne Editor to tidy-up monophonic violin and viola parts, across many musical genres. It's like a magic wand
It's nearly always convincing, with very few audible artefacts. It's tolerance to typical fiddle performance nuances is great....except where multiple strings are being 'slided or bent' concurrently.
I would expect fretless bass, being generally monophonic, would also give a fair chance of success.
But, should anybody ever need to ask - perhaps unsurprisingly, I have never had much success with Melodyne on pedal steel guitar or Dobro/lap steel.