I really haven't watched anything from this guy, but that "Fighter 4/Magus 3/Monk 2/Ranger 1/Bard 2/Vivisectionist 2/Stalwart Defender 2/Dragon Disciple 4" clearly is an attempt to throw as many classes as possible into the same build and still obtain a functioning character that you can finish the game with. And, honestly, I think it works.
However, usually, taking 20 levels in a single class is more than enough to finish the game on any difficulty. For martial classes, the occasional 2 to 4 levels dip can sometimes be beneficial, but far from essential. Take the famous Haplo's tripper build (
https://rpgcodex.net/forums/index.p...aker-builds-and-strats-thread.124160/page-113): it's a Vivisectionist 12/Monk 1/Sword Saint 1/Archeologist 2/Dragon Disciple 4, but it works even as a simple Vivisectionist 20. Starting from that, then you can complicate it as much as you want (the best one, according to my taste, is just something like Vivisectionist 18/Monk 2).