All systems have the potential for corruption.

A representational "democracy", compared to other systems, has the least potential for corruption.

If unions do in fact give workers more representation/power, than game theory says it is a worthwhile trade off, as we expect average worker gains to be net positive of any corruption losses.

Corruption usually happens on jobs with low pay that noone really want to do but does it anyway due to a good cause. But once in a while you get people that are driven by power and glory, and if the position holds any power they will use it to their own advantage.

Direct democracy has less of a potential for corruption than representative, especially non nested (separate tires) party based representative democracy.

There are tricky problems here as well.

In direct democracy, a decision still needs to be made on what to vote on. As an example, if you look at California propositions, these aren't necessarily all that well drafted. Often propositions are created by business interests and approved by people coming out of stores who aren't thinking very carefully about what petitions they sign.

And after that point, you're putting an algorithm is in charge of making the final decision. This isn't necessarily better than having a leader in charge, because the leader can maybe think of creative compromises and the algorithm can't.

If you study voting systems you'll see they all have flaws, and they are all very simple-minded algorithms compared to people. Whether they actually capture the "will of the people" is impossible to say since the "will of the people" isn't really a thing. There are just people with a lot of different opinions.

Of course not. But democracy is better than no control. A union can fight against things like unpaid oncall or crunch or other abusive practices.