I had problems with thumbnails being generated in the file browser for the longest time and finally figured out what was going on. For some reason, Nemo was using a dogshit xplayer video thumbnailer for creating thumbnailer that had very poor performance (would take forever to generate a whole folder of thumbnails) and would straight up not work with/ignore several video formats. I uninstalled that shit and Nemo fellback on using the ffmpeg thumbnailer and now everything is perfect. Fast thumbnails and thumbnails in every format I've seen so far.