Banks use tools as a store of value by holding some assets in reserves, for various economic benefits. Although, adopting Bitcoin in such capacity, would have it's pros and cons.
What is the cons of making bitcoin a reserve in the national level? I do not see any cons about it. Only what I see are pros because bitcoin is different from the fiat the countries are using because bitcoin is deflationary. Bitcoin price may fall in short period of time but it will later increase, get to all time high and still increase more.
There are cons, maybe on the individual bank level, they have to go through long and strenuous regulatory protocols to keep Bitcoin reserves directly which is why some of them feel better to invest into the spot ETFs, since the ETFs operate on already established regulatory guidelines.