Its an extremely common thing here with rescued street dogs here in singapore, and generally it is very useful should a dog run away - providing longer term, very good high quality tracking.
I'd say, considering the size, its about as 'safe'/'dangerous' as any other tag. If your dog is the sort to chew on things like that and swallow it its a bad idea. If you feel your dog is a flight risk, its probably a very good idea.
I've not heard of any cases of it being dangerous, but there's a good many success stories for its use