Recently been playing with this. Using FFMpeg to generate videos from a series of stills, I assumed a frame rate of 1 and a fixed video length would be suitable... Turns out a lot of players are very particular about how they like their files to be set up. Windows couldn't open the file, VLC could, Google couldn't generate thumbnails, but could show the video. Playing them on a Pi lead to more fun and games.
In the end I just encoded them the 'correct' way, but it was eye opening to the wildness going on in video files. I just assumed I would be able to set a duration, a frame rate, and things would "work".