Good to have some things from the video cleared up. Great to hear that you're working hard to iron out all of those problems and that a heap of the problems that people saw were actually pretty neat features.
It sounds as though fire damage is super-effective against everything but stone. Personally, this seems a little odd. Yes, if you get get immersed in a substance which is on fire and can't be put out, you're probably toast, but a smaller delivery system, like a fire arrow, would pose very little threat to a person or a horse; much less than a specialised anti-personnel arrow. Likewise, catching on fire, but not being covered in a flammable substance is a pretty easy situation to deal with unless you're inside a burning building and can't escape.
Fire is a great addition to the game and very important in sieges, both for the attackers and defenders, but I'm not sold on it being super-effective against any living thing. As someone who works with high temperature furnaces, I can tell you that surprisingly little protective equipment is needed. A woollen jumper or a thick leather coat can completely protect me from brief exposure to temperatures that do melt rock. A woollen gambeson would offer significantly more protection. Even being doused in burning oil, I think you would survive for quite a long time before any deleterious heat penetrated.
Other types of armour, like metal scale, lamellar or plate armour, are completely unabsorbent and boiling oil might well run off them without imparting much heat at all to the underlying garments, which could conceivably be flammable, though certainly often weren't.
Realistically, it's a very different type of damage to blunt/cutting/piercing and depends on many more factors. I hope it's going to be a great and tactical feature, but not a catch-all super-weapon damage type in Bannerlord.