It's only dangerous if you don't know what you are doing. Which, I suppose, is 99% of the American population.
For operating a generator and backfeeding a panel, there are two rules;
1. Never connect the generator (running or not) to the panel without turning off the Main Breaker FIRST.
2. The Main Breaker shall ALWAYS remain off whenever the generator is connected to the panel.
Yes, it is true about not wanting to "backfeed" the grid and zapping linemen. What hasn't also been mentioned is that the generator will be severely damaged or catch fire if utility power is restored while the generator is connected to the panel (running or not) and the Main Breaker is On/Connected to the grid and Utility power is restored.