If it was totally safe, then the manufacturers would most likely have used it themselves.
It is not like intel CPUs where they sell the same one running at different speeds and charge different prices.
Problem you have to keep in mind is that although your device may appear to be working perfectly, you may be reducing its working life big time.
If that is from 10 years to 5 then who cares, but it may be 10 years to 6 months.
Personally I would not risk it. From what I have read from PC world, manufacturers are usually able to spot overclocking as well, so bang goes the warranty.