Assuming you believe in democracy, can’t democracy prevent abuses of power?

I do believe in democracy, but proletarian democracy, not bourgeois democracy. Democracy under capitalism will always been one dollar, one vote. The wealthy own the media, so they frame the public discourse. The wealthy fund politicians’ campaigns, so they have the politicians’ ears. They decide who makes it to the primaries, so by the time you get vote, your options have been preselected.

Maybe we agree that bureaucracies will squander wealth

I don’t agree that that is a universally true. That’s the ideology of neoliberalism under which we currently live, though. They’ve spent the last fifty years under-funding public services until those services under-perform, then blame the poor performance on the fact that the service is public, and then privatize it. This was an inevitability, though, because the capitalist class already owned everything else, so of course they were going to privatize whatever public commons was left.

Aren’t the means of production limited by demand?

I don’t see the relevance. What would be the point of producing stuff no one wants?

Like they have to make something people really want? So what’s an example of something bad that they couldn’t do in socialism?

I’m still not quite sure what you’re getting at, but a lot of what we want is thanks to advertising. Corporations spend ungodly amounts of money on advertising in order to influence our desires. Smoking wouldn’t have become pandemic if not for people like Edward Bernays, the father of advertising & “public relations”, A.K.A. propaganda.

a ubi from the revenue solves that

I don’t think UBI under capitalism would solve much, because companies would inflate the prices of their goods and services to capture much of it.