What is the nature of a monetary crisis?

OECD countries are increasingly conducting an ultra-expansionary monetary policy, and this raises fears of a monetary crisis caused by the excess money supply. But what type of monetary crisis could be caused by this excess liquidity? In the past, it was very high inflation combined with a sharp depreciation of the exchange rate; this still happens in emerging countries, but no longer in OECD countries; Currently, it is the appearance of bubbles in financial and real estate asset prices as the money created by central banks is invested in other asset classes. Real estate bubbles in particular may have very negative social effects by making it difficult for young people and the middle classes to gain access to housing; The most drastic form of monetary crisis would be a rejection of public currencies, given their excessive quantity, and a switch from transaction money to private currencies (private cryptocurrencies), which would eliminate the role of central banks and seignorage (the inflation tax, the ability to monetise public debt.) )