Depend on intention when using it, indeed there is no problem with money fiat as long as you use it in your country. You are freely using it and it will be accepted in all aspect when you pay anything. But, could you imagine what will you do when you use it in another country? You can't use it for sure.

Also, when you have a brother/friend who lives in another country and you will send money to them using money fiat, how many days the money will arrive? At least it can takes three days right?

This is the matter of scallability, it will be different when you use crypto currency. As long as the merchant use it you can pay anything with it as long as you buy something in different country with a low fees and also you can send money to your brother/friend who live in differrent country just a few times and low cost. This is why we need regulation form the government and seem like there is no impendence when they regulate it.