I think the most notable differences compared to the NS40 seem to be that NS50 drives can at least 'read' UHD discs (well they can read the disc structure - something that NS40 drives can't do - but as far as I know they are not UHD Certified so they can't actually play commercial AACS 2.0 UHD movie discs), plus a revert back to the uglier plain bezel instead of the nice 'premium' bezel that NS40 drives used.
Burning wise, both families are good burners with very similar results. From my experience, at least for the MIDs I tried it on with (mainly Verbatim CMCMAGBA5-000 and Infomedia INFOMER40-000), the NS50 produces perhaps slightly better quality burns than the NS40, but for other MIDs the story could be different, so I cannot really generalise. The relevant NS50 and NS40 threads would obviously give a much better picture of the two families for a wider range of media.
Other than that, I don't think there are any notable differences in R/W parameters in between the two families (if any at all), but I haven't really done a detailed side-by-side specsheet comparison of the two families... for a start finding a PDF specsheet in English for any NS50/55/58 drive seems to be a rather futile exercise