This is true, but unlike any other workplace, if your union is filled with corruption and nepotism you can vote to change your union leadership or even to disband it entirely and replace it with a new union under entirely different leadership, operating under new rules written to specifically address the problems with your old union.

Unions at least give you the option of improving the situation if the vast majority of union workers agree that there's a huge problem.

Face prison and death? No thank you, no job has ever been worth that.

Thinking that you're going to fix deep corruption via democratic process... You realize that this doesn't work in mainstream politics either, right? Only in the movies.

As for corruption, a strong democracy is highly effective against it which is why corrupt states try so hard to weaken or eliminate Democracy where they can. Democracy is absolutely a threat to corruption.

The US does seem to have slipped some (https://en.wikipedia.org/wiki/Corruption_Perceptions_Index) and there's no surprise that there have been some very worrying efforts to weaken our democracy recently. Most Americans think our democracy is in danger. This would probably be a good time to fight for a stronger democracy so that we're better able to deal with corruption.