When dealing with videos of different length, it takes some work to sync the audio and video. If there’s a ten second logo intro in one video, but not the other, I can add a delay (or inverted delay) and it seems to work. But what about videos that have extra content in the middle, like an entire opener that is in the dub, but not the sub? I’m guessing it depends on where you get your content.
If your content is clean, it’s easier to remux.