Neodymium magnets are easy and cheap to come by so
price is not a factor. These magnets are dangerous and care should be used handling them. Here are the reasons:
a) They shatter easily. The force of two magnets attaching to each other is enough to break them and cause shrapenel to fly.
b) They can seriously hurt someone. Companies warn people about the power of the little magnets. Bigger ones could accidentally break bones unless care is taken.
c) Once they are attached to something they are really hard to separate. I am not really sure if this would be an safety issue but you would have to be design the robot with some way to easily deattach the magnet.
d) Aparently they can affect pacemakers and other medical devices.
e) They can ignite accidently and burn very fast like magnesium does.
More Safety Tips Wow
Even More Tips
The Japanese have used these magnets with a lot of success in their sumo robots. They actually fought with their robots upside down.
Here is another link to more information:
Another supplier/Faq on magnets