> It's true or it's a part of nikon business policy?
If you think it is part of Nikon's business policy, go ahead and use any flash you want. If they are saying this to sell flash units, why pay any attention to them?
Hope you have money for a spare F100 however once the blue smoke pours out.
> Can I use my Metz with my F100 without risks?
I did not check, but probably not with an F100 or a Canon EOS or a Hasselblad H1, or any other camera that is not purely mechanical. I have a case of Vivitar 283s, none of which I would trust for any electronically controlled camera. They were built during the days before digital electronics and could have trigger voltages as high as 600V! Nikon says most of their 35mm cameras can take 12V, while Canon recommends no more than 6V.
For a LOT more information on the subject read:
If you choose to think this is just a marketing tactic, go ahead and ignore the warnings. If you believe that camera manufacturers and flash makers have dropped their voltages 100-fold just to sell new equipment, and screw you out of your money, by all means use your old stuff. No kid really understands mother's warnings until they touch the stove for themselves. Try it, the camera can always be replaced.
When you see blue smoke, inhale deeply. It will be the most expensive substance you will ever snort. Enjoy the moment. However, it will be up to you - not Nikon - to take responsibility for such behaviour.