I think it’s important to distinguish the different kinds of monetary unions. The US Dollar was initially a currency union amongst all the U.S. states, but it became a national currency once the states were bound by the federal government. That’s relatively easy to manage because of the political unity.
There have been monetary unions amongst distinct nations before and they haven’t lasted very long (about 50 years max). They generally get disrupted by financial crises or wars. Some examples are the Latin Monetary Union and the Scandinavian Monetary Union. The Euro is an interesting ongoing case.
For the reasons I mentioned previously, I wouldn’t pay too much attention to articles about the BRICS forming their own currency. I’d love to see a serious argument that says China will cede their monetary policy to the interests of the BRICS.
I think the more realistic concern is for them to denominate their trade in their own currencies rather than USD.