Hydrogen generators

I have heard and read some things about hydrogen generators for cars. These are simply water plus electrical current plus a tube or hose that leads from the hydrogen to the air intake. My questions are: Do these systems do all they claim to? What are their downsides? Why do we not see more of them?

“Why do we not see more of them?”

Because they are all SCAMS. It takes more energy to separate out the hydrogen than you can get back burning it. You can’t brake the laws of physics.