Living on Mars cannot be considered entirely risk-free, in particular during the first few years. There are a number of elements that could pose a problem:
Obviously, Mars One will extensively examine and trial-run all elements of the mission beforehand to pre-empt any mishaps – especially the settlement's critical parts.
Living on Mars is comparable to getting by on i.e. Antarctica, and provides similar challenges. However, the South Pole now has a number of very advanced, large research stations that boast a great deal of modern facilities that provide a good quality of life. On Mars this development still has to be kick started.