They are safe. You will have to use premium unleaded, no skimping.

They work by taking some of the 'slack' out of the programming from the factory. Due to differences in the way people maintain their car, what grade fuel they use, emissions concerns, etc. The factory's programming is very conservative. Aftermarket chip manufacturers modify the fuel & timing tables, and may increase the rev limit.

How effective the various chips are is a matter of debate.