That risk exists independent of MMT. We could deficit spend an airdrop of a million bucks to every American with a bill today under modern economics, and still destroy the economy with hyperinflation. We could borrow trillions in a foreign currency and face hyperinflation risk today.

As I understand it, MMTs actionable insights range from issuing less/no debt for the same government spending, offering jobs to the unemployed and underemployed through a federal jobs program, and tinkering with government spending /taxation as a tool for controlling inflation (as opposed to interest rates as a tool for controlling inflation).

Note that regardless of your economic beliefs, these are all still subject to the same political system we live in today. Most of the worst case scenarios people imagine with MMT look like "we gave someone the power to print money and they printed too much money". But in the real world that's a problem with autocracy, incentives, and feedback loops. If we keep autocracy out and still monitor/alter incentives and feedback loops as needed, it's hard to imagine MMT having a massive downside risk any moreso than existing economic orthodoxy.

Modern Monetary Theory is the idea that the government can go ahead and just pay all its debts with IOUs, which is all cash really is anyway. You don't need taxes to pull in cash and dole it out, which just complicates things. You might do it anyway, to control inflation, but even orthodox Keynesian theory says that a little inflation is a good thing (since it encourages people to invest rather than put their money under a mattress).

The idea is to pump enough cash into the economy that employers (or worst case, the government itself) will find jobs for everybody who wants to work.

It's kind of the flip side of the inflation-averse Chicago/Austrian school of economics, with mainstream Keynesian economics in between. The Chicago school is strongly associated with the right wing, and MMT is receiving a lot of attention right now from the left, who want to use it to fund a lot of progressive spending policies.

Is this what China is doing? I often wonder about whether there are real downsides to just printing money as long as the rest of the world buys into your currency. Seems like printing money fails only when there is lost in confidence. But as long as the confidence is there, maybe it can just continue working.

There's no reason to assume that there's real estate asset bubbles, we don't have overlending/unqualified lending. You could argue that there's scarcity in real estate, driven by local governments restricting the supply of housing through historic regulations and single family housing. And that as a result, the only time of private investment that can earn a return is luxury housing which fails to be affordable for a large segment of the population.

MMT doesn't say anything about what you use the funds for, but, if you're a Keynesian then you need to keep your eye on productivity and growing GDP, and you need to worry about inflation. Depending on your politics then you could spend on public assets like infrastructure, green energy, etc. But most MMTers suggest a federal jobs guarantee that would create a buffer for people that the private market will not employ. The job guarantee would employ people to do all sorts of public projects to anyone willing and able to work. You just have to be careful about inflation. If you spend money on steel and concrete and you put people to work in construction, you're going to suck up resources that the housing industry needs, and if you don't have the resources you're going to get inflation. Ideally you'd spend money in areas where the private sector doesn't want to or cannot deliver low costs(e.g., education, environmental cleanup, healthcare.)