Well, it seems half the people criticising MMT say it's a vacuous re-statement of how things already work, and the other half say it's wrong and discredited :p.

What does it actually advocate that has been discredited? I get the impression MMTers want to erase the distinction between central banks and the government, and basically run everything with fiscal policy, letting the government decide how big its deficits are instead of effectively the fed deciding. Is that right?

I mean, that would work in principle, right? It would just be a bad idea because the decision is best made by an independent entity rather than the government, which has short-term incentives that may lead it to make bad decisions about spending (usually: too much).

And maybe it would be a bad idea because interest rates are a more reliable lever for controlling the money supply than tax and expenditure rates? I mean, if that's the case, it's not super obvious.

And since these things haven't been tried, I don't suppose it's those things that have been discredited.

From what I see, people are saying MMT has been discredited because large government deficits have led to high inflation recently. But this seems off to me - MMT doesn't say infinite deficits are possible, they're saying the deficits are limited by inflation. Which is true and we're now seeing the consequences of exceeding that limit.