Regarding them been dangerous. The rules Roar have enforced for the packs to pass there tests are pretty strict as far as I can tell, this includes over-charging them to see what happens and a drop test. With these tests in place only packs that will be legal will be the safer ones. Most have seen what happens when a NiMH is over-charged and its not particularly safe
Regarding the NiMH/Lipo performance debate, I cannot honeslty see a performance disadvantage to running NiMH as I feel any off-road car can be over-powered as it is, unless you are currently running the quickest motors you can buy and still wnat more power you have no reason to complain on this front. Therefore cars that can only run NiMH will not have a disadvantage and a new car can be properly developed to allow the use of Lipo or both cells.