Bank of Japan's balance sheet is larger than Japan's GDP.

https://finance.yahoo.com/news/graphic-bank-japans-balance-sheet-083941933.html

When a central bank is bigger than its own country GDP, it is facing tremendous stress in managing its assets because of the different risks it faced in different markets.

What does this mean for the market?

This means the stock market is artificially being held up and the bond market is also flushed with fake liquidity.

When there is a global recession, BOJ will suffer from a massive dilution in asset values.  The BOJ will also suffer great losses in its bond assets in an interest rate rising environment because the bond price and interest yield move in an inverse relationship.

The BOJ will be cash-strapped when it is holding a lot of assets and will face cash flow problems when it is needed to perform its origin duties as a central bank.  It cannot cash out easily because it will disrupt the stock and bond markets.