Modern Monetary Theory tells us that the important limits are "when you run out of Labor" and "when you run out of resources".

So what nations are facing low unemployment, and what nations are facing resource shortages?

I'd add an additional criteria of: "when inflation is out of control". I don't know about other central banks but The Federal Reserve and Bank of Japan have been trying to raise inflation for years now, interest rate cuts are not as effective when inequality is high because it just leads to asset bubbles. It's time for fiscal policy to direct spending to productive areas.

So MMT says so. So what?

We don't automatically take what every non-mainstream economic theory says as gospel. You need a bit more reason for us to accept this one than "MMT says so".