Japan's debt has been sold at generally less than its inflation rate for a decade, making servicing costs essentially below zero over the last decade. While that may not be stable, you can't get into a debt spiral with a negative interest rate.
That said, they have been running large deficits for a long time which is a separate issue.
Perhaps, it's an odd situation. If you exclude the Bank of Japan's holdings Japan's debt to GDP ratio is ~70%.
It's as if the FED used QE to increase the money supply and ended up owning most US public debt. Sure, on paper it would look bad, but there would be nobody to pay back that debt to.
That said, they have been running large deficits for a long time which is a separate issue.