I suppose it depends on what you mean by safety.

NiMh batteries are not suitable for lights you intend to store for a long time because they self-discharge very quickly.

If you have lights that you use frequently AND you use them correctly, NiMh batteries not only can be safe, but they can give great performance and lower costs.

Typically NiMh batteries retain the voltage they supply much better than alkaline batteries. Compare the graphs between alkaline and NiMH in these two great threads on CPF:

Alkaline Battery Shootout
NiMh Battery Shoot Out

So in general, NiMh batteries work well for high output lights. However, you must be careful not to over discharge them and careful when charging them. The best way to charge them is individually.

Because of their high capacity, this type of battery can be very useful, but they also need to be used a bit more carefully than alkaline batteries.

-john