There should be nothing dangerous for the generator itself about the frequency,
but you would have to be careful about anything connected to the generator...
A simple bulb (for example) will be fine.
However the first thing many electrical appliances do is pass the current through a transformer, and then through a rectification and smoothing circuit to get Direct Current.
These power supply conditioning circuits will be designed for a particular input frequency and will probably not work or even be damaged if supplied with frequencies significantly different from that for which they were designed.
You can design a generator for any frequency, However in the world the power systems are usually at 50 Hz and 60 Hz, So you should be careful about this as the allowed frequency variation for the load is 2%. Also if you are driving it at very high frequencies such as 5kHz or 1000 Hz its machinery might get damaged.